It had shit level design and felt like a rushed, unpolished game. The Japanese version, anyway. The US one, I barely acknowledge its existence.
See, in SOR2, there is a difficulty curve. Every enemy is one step more dangerous than the previous one. Also, the enemy placements are much more varied, you have always different combinations of enemies to deal with. Even the generic Galsia/Donovan spam has some variety, be it a random Signal, a Koksal Baba, a pipe wielding Donovan, a biker, or a mid-boss mixed in.
BK3 has like a dozen generic punks, and they all attack in masses. None of them pose any specific challenge, such as some move that you have to watch out for. There's just a lot of them and that's all the difficulty.
SOR2 recycling bosses actually comes out as a good thing in this regard, since it can spice up normal fights with the occasional mid-boss. Speaking of mid-bosses, SOR2 not only has more, but most of them also have unique introductions of a sort, be it Electra shedding her coat, Ninjas jumping off the pirate ship net, the duel with Big Ben, or the one-on-one duel with the karateka in the back of a minivan. Also the bosses being reused works well, since you then know that shit is serious, while BK3 just throws 10 generic punks and a slightly faster Galsia or Big Ben at you.
So SOR2 has better enemy placement / level design, but also more detailed pixel art (BK3 tries to go for more realistic style instead), and way more audio. And I don't mean the music, I mean the sound effects. SOR2 has like 9 different punch sounds that all feel different, BK3 has maybe two.
Even with all that, I'd say BK3 is very fun, up until stage 4. The train level kills the game. I also hate how Electras take an eternity to stand up (not fun on the stage 5 elevator).
Also BK3 has shit AI and enemies cannot be hit if they are partially offscreen.