Stating an opinion and stating it again.... wow.... and based on what information? OP I use buckets and they are fine. I like them for the same reason as post #1
Personal experience with using them... I used one for my very first brew, and for another brew before I had picked up more carboys... Both times, PITA... Compared with carboys at least.
I'm migrating over to use kegs for fermenting (~5 gallon corny and Sanke kegs) and aging now. With the significantly smaller opening (when pulling a hydrometer/taste sample, less chance of something negative falling in. Just need to get a few more of the orange caps for the Sanke kegs (picked up one today, needed to make sure it would work before getting more) and I'll be in business.
Fermenting in either keg has more positive aspects than either a bucket or even carboy. Although they share more with carboy's than buckets...
1. Zero light penetration. Plastic buckets are not 100% light tight.
2. Zero gas penetration through the fermenter's materials. Plastic could allow gas through, given enough time. Probably not going to be in there long enough, but zero chance is better than a chance.
3. Small top opening similar to a carboy. Benefit over buckets.
4. Easy to pull out a sample via turkey baster or wine thief (match with carboy).
5. Racking from them will be easy. More so than buckets or carboys. I've already done this from the corny keg, the racking cane holds itself upright, without any additional hardware.
6. Zero chance of scratching them using cleaning methods available (keg cleaner tool).
7. Life span... Kegs can last for much longer than either buckets or carboys. It takes a LOT more force to damage a keg than either a bucket or carboy (glass or PET)...
I'm sure I'll find additional items as I use them over the coming years... Only possible negative is that the capacity is just over 5 gallons. But, that is negated by using fermcap and/or blow-off tubes. I also plan on making a fermentation chamber this year. I'll be able to fit more of these into the chamber than either buckets or carboys. Since they have a diameter of no larger than 9", and it's the same for the entire height, it will be easy to get the size chamber I want. Or build one to house them.
People tend to love the what they're fermenting in... Until they find something better... I started with the bucket (as already mentioned) and immediately disliked/hated it. Carboy's were a much better option for me. Since figuring out how to use kegs, that will become my vessel of choice moving forward.
The only reason I used a bucket for the melomel, was due to being able to leave it without the actual lid on it (used a sanitized towel, secured to the top)... That was only for the first week though. After that, it went into a glass carboy. Not sure how long it will be before I do another melomel... Need to see how this one turns out in X months before I actually decide.
Way I see it, use what you like. But, you can use almost anything you want, as long as it's made from safe materials. That means either glass, the correct type of plastic, or stainless steel... Don't segregate one size/type of vessel to just one task. You can easily use 5 gallon carboys as primaries. No need to use them JUST for bright tanks/secondary vessels. You don't HAVE to have over a gallon of head space in order to ferment. Granted, there are some yeasts that you'll want the extra space for. But, you can also do other things to reduce that need.