I used to be a hardcore raider in FF11 and WOW, so this particular boss mechanic is one that I really capitalized on when creating my boss fights. I think creating "phases" dependent on the boss's HP, where they gain/lose abilities and change up their tactics keeps a boss battle engaging and fresh as the player continues to push through their HP.I'm a pretty big fan of bosses that, as their HP drops, go from being overconfident and lazy to desperate and dangerous, and this gets reflected in how brutal their abilities are. Not saying every boss needs a FINAL FORM, but they should get nastier or change their tactics a bit as their HP goes down to keep the battle interesting.
I am a pretty big fan of giving bosses an "enrage" as well. For example, in my game Eternal Twilight, the 3rd boss of the game (the infamous Minerva) has a "soft enrage" which increases the damage she deals by a small percentage every turn that you are in battle with her. Every 6 turns she does a massive nuke on the party which is pretty easily survivable the first two times she does it, but because of her soft enrage increasing her damage, surviving the third one is most likely a party wipe unless you debuff the hell out of her.
I personally like long boss battles, because bosses tend to be my favorite part of playing RPG games, buuuuuuuuuuuuut, for the love of god, make that long battle engaging and evolving as it goes on. This, I think, is a hard balance of figuring out how to not make the boss die too quickly and how to make it a long, but challenging and fun battle.