Just out of curiosity, something I see mentioned from time to time was that unearthed arcana was considered bad or unusable. I remember reading it back in the 90s and liking some stuff about it, but I never used it in 2e (which I was playing at the time). I probably didn't have as critical an eye back then either and, since I never used it, may not have seen all it's flaws. I do recall thinking that it was a little unfair the cavalier could slowly increase some ability scores and tha barbarian seemed to need ridiculous amounts of xp and couldn't adventure with spellcasters (or something like that).
So for those who did use it back in 1e days, why was it considered so bad?
Some of the new spells were highly overpowered and flat-out broken as written. Others were nearly useless.
Ditto for the new magic items.
The new classes were pure power creep (Cavalier) or hopeless (Acrobat) or unplayable as written (Barbarian). The new abilities for existing classes e.g. Fighters' weapon specialization were also just power creep.
Comeliness was pointless; just another place to dump a bad stat.
The new roll-up method was more pure power creep.
Some parts e.g. the expanded treatise on polearms were of interest only to a very select few and kinda wasted on the rest of us.
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EDIT to add: Forgot about the new PC-playable races. All bad. Dumb ideas from which the game has never recovered.
Also forgot about the relaxed demi-human level limits and greater class options; those were good, and we'd largely already done that before UA came out.
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Put that all together and most of the book was, as written, a big disappointment to someone running original 1e.
There's a few gems in it, though, if you dig and are willing to do some kitbashing. Cavalier (or Knight) as a class concept is good, just needs rewriting. The Cavaliers' percentile-increment system for stat improvement is brilliant and should have been applied to every class. A fair number of the spells and items are salvageable with some rewriting and adjustments. Weapon spec. works great if it's toned down a bit and the benefits made incremental across levels rather than coming all at once. And so on.