Yeah, it's a mixed-bag. A lot of it depended how hard it was to copy the original.
Things without custom chips (Double Dragon, Bubble Bobble and many older games), you could get a pretty good copy.
When you get to 90's stuff like CPS1, Mortal Kombat, they were doing the video in their own way using masses and masses of GAL chips or even FPGAs/CPLDs. Some stuff you wouldn't notice, others there'd be things different like scrolling effects.
The CPS-dash stuff had an entire extra Q-sound DSP board and I don't think anyone attempted to reverse that, so you had a right mish-mash of attempts - you had The Punisher with Final Fight FM music, some had an attempt at badly sampled music, I think?
The biggest problem I see with the newer stuff it the awful build quality - you can see bodge resistors everywhere, hardly any capacitors, and saying 'there's no custom chips to fail' is misleading as there's tonnes of other stuff you wouldn't fancy replacing. SF2 bootleg was literally a £5 PCB and you'd be mad to pay a load more for one even now.