If you are an experienced hiker, with the right equipment, stamina and a head for heights, this trail and its risks should be no problem. However, portions of this trail are rated T5 for a reason - they are difficult, and require a high level of experience and skill. It is not for everyone and if you have any doubts about your ability to deal with the risks of high vertical exposure on both sides while hiking a narrow path, do not attempt it. I have seen videos online of people hiking this thing with no preparation whatsoever, and they were lucky. They made it.
I prepared by hiking up to Augstmatthorn from Habkern, Interlaken and Harder Kulm a dozen times before attempting the full trail, and only then when the weather window was wide enough to accommodate eight to ten hours without rain, storm or serious wind. It is not a trail to be 'conquered' or entered into lightly. There are dozens of other amazing ridge trails in the area (Brienzer Rothorn to Turren, Brienzer Rothorn to Soerenberg/Planalp/Brienz, Schynige Platte to Grindelwald First, Niederhorn to Gemmelalphorn/Habkern, and more) that all offer incredible views and short and long choices when it comes to your time on the trail. This is an unmonitored, unofficial trail with portions that are white-blue-white Alpine Trails, meaning the difficulty level is very high. Accidents happen up here, and there are memorial markers in a number of places along the way to remind you of that fact. My condolences to those who lost loved ones up here. Their markers remind me every time I pass by. I have spent enough time up here to know that most people on the ridge know what they are doing, they have researched the trail and trained for it and they too approach it with respect and caution.
DO NOT UNDERESTIMATE IT. That really is the best advice I can give. 1100 vertical metres of total elevation gain (direction Brienzer Rothorn to Harder Kulm) doesn't sound a lot, but when you have to climb nine peaks in a row, each around 100 to 150 metres up and 100 to 150 metres down, it takes a serious toll, and requires serious effort and stamina to complete. About half of those ascents also require actual climbing using all fours, not just walking. That often comes as a surprise to many hikers up here. There is no margin for error up here and there is no easy or short way to leave the ridge once you get going. At Ällgäulücke there are two exits from the ridge, down to Oberried am Brienzersee at the lake side (or back to Planalp and from there down to Brienz) and in the opposite direction across the meadows to Kemmeriboden. Depending on your direction, this does offer a solid place to consider if a: the weather will continue to hold for the remainder of your hike (3-4 hours additional in the direction of Harder Kulm; 2-3 hours additional in the direction of Brienzer Rothorn) and b: if you have the energy or supplies left to complete the ridge. And remember, just when you think you are done with the ridge, you still have to get off it and ALL the exits except for Brienzer Rothorn and Harder Kulm will take at least two, sometimes three hours, of punishing steep downhill. There's a good but steep route from Blasenhubel down to Oberried Am Brienzersee, that's about 2-3 hours from the peak to the lakeside train station. Augstmatthorn and Suggiture also offer multiple exits from the ridge down to Niederried/Oberried Am Brienzersee or down on the western flank to Lombachalp and Habkern.
Take your time. Make sure you bring enough water (at least 2 litres each person, 3 if you can carry the weight) and food for a good 5-7 hours with no place to replenish supplies. Hike to Suggiture/Augstmatthorn from Harder Kulm, Habkern or Lombachalp or take the train or cable car up to Brienzer Rothorn and take a long look at the whole ridge. Once you see it in person, you'll get a better feel for its length and whether or not it looks like something you want to do. Never attempt it in heavy rain, or if heavy rain or storm is forecast between Brienz and Interlaken within the 7-9 hours it'll take to complete. Wind is of course a major factor, and that too must be considered on this very exposed ridge.
I waited until almost all the snow had melted in the area along the ridge before attempting it, and still watch the weather carefully before every pass. I have completed the ridge over 20 times now in varying conditions from cold and wet to foggy to super clear and sunny. All present their own problems, but rain and muddy conditions are the most difficult I've encountered, so pay attention not just to the current/forecast weather, but also to the weather in the days preceding your hike. I have completed this ridge trail in both directions (from Brienzer Rothorn towards Harder Kulm and from Harder Kulm towards Brienzer Rothorn), and starting from Brienzer Rothorn is in my opinion the easier way. It is almost half the vertical distance than going the opposite way (Harder Kulm to Brienzer Rothorn is an approximate total of 2,000 metres in vertical change, the opposite direction just over 1,000 metres of ascents). The most challenging part of these ascents either way, is they are numerous, about nine distinct ascents each way. That is very tiring on a good day, and requires serious attention to the pace with which you approach this trek.
Also consider your timing, as the last transport (cable car/train) down from the Brienzer Rothorn end is between 17.15 and 17.40, and at the Harder Kulm end the last funicular down is around 19.30/20.00 through most of the season. You have to factor in the time it will take you to get there without rushing to make the last transport. I usually plan for a safe (well-paced) nine to ten hours from Interlaken to Brienzer Rothorn all in and seven to nine hours all in from Brienzer Rothorn to Interlaken/Habkern.
NO CAMPING ALLOWED. In recent weeks I have counted dozens people who are intent on camping along the Brienzergrat to Hardergrat trail. I usually see one or two groups hiking with full camping gear every time I am up here. One person contacted me on Instagram, inquiring about camping along the ridge and I explained that it was not allowed yet still I found him camping there the following weekend. It is illegal to camp ANYWHERE along this ridge. The ridge is part of a protected area for Steinbock at the Augstmatthorn end, and from Tannhorn to Rothorn the ridge forms the edge of the Entlebuch Biosphere. There are clear signposts at both ends of the trail: No Camping. Please consider the wildlife and the environment. The more people camping here, the less the Steinbock will return and the ridge becomes spoiled for all of us. Please follow the rules!
If you have any doubts or concerns (weather window potentially too short, experienced friends can't make it, timing to reach the last transport off the mountain is tight, whatever) wait for another opportunity. Make sure you hike with someone who is experienced either with this ridge or with any high alpine hiking. Pace yourself and work out your exit options if weather or other unforeseen conditions mean you have to exit at Augstmatthorn or Ällgäulücke. More than anything, don't fuck around. Don't mess with the trail, don't go chasing Steinbock off the trail or seeking an amazing Instagram shot on portions of the ridge not on the trail. It isn't worth it. I have seen people running and jumping down Augstmatthorn to start this trail, and maybe they are able to do such things and sustain it for 6-8 hours. I can't. I know my limits. Respect the ridge, and your fellow hikers. Passing is often difficult up here, so be courteous and patient, no-one should feel hurried to 'get out of the way' as there often isn't a place to get out of the way. Do not leave any rubbish behind and if you are able to, pick up anything other people have carelessly left behind. Make sure you sign up for a Rega Patronage (Swiss Alpine Rescue Service), download their app and read their safe hiking tips before you start (before you start any alpine hike or climb): rega.ch/en/news/news-from-the-world-of-rega/detail/well-prepared-for-an-excursion-in-the-mountains.