Hmmm, so I think a premise is mandatory, to clarify some aspects which I honestly took for granted.
Raspberry Pi is a series of small single-board computers developed in the United Kingdom by the Raspberry Pi Foundation to promote teaching of basic computer science in schools and in developing countries. Its purpose has never been "raw" computing power: the idea was to create a computer that wasn't just cheap, it was almost disposable, with a price tag so low that kids wouldn't be afraid to carry it with them or to hook the Pi up to other hardware and build their own gadgets.
In this regard, I really recommend reading this interesting article, to "contextualize" the Raspberry Pi avoiding false expectations:
https://www.techrepublic.com/article/inside-the-raspberry-pi-the-story-of-the-35-computer-that-changed-the-world/
Eben Upton said:
"The idea of a breakable computer was important to us, it had to be at a price point where it didn't feel that you were risking the world by connecting wires to it."
So working on Raspberry Pi means in most cases "compromise" as a counterweight to its extreme affordability, and the best way to exploit it is to keep in mind its original purpose (and that's also great part of the fun!). So, for example, equipping it with extremely expensive accessories could nullifie the meaning of its use. About emulation: obviously RPi is not able to run at full speed latest releases of MAME or things like TeknoParrot, very demanding in terms of hardware requirements (a credit-card sized board capable of it is probably LattePanda which, however, costs as much as 10 RPi and yet it lacks some unique features that are possible on RPi, as in the tutorial).
Someone then may not consider RPi a real option for emulation and that's OK, but recent fortuitous features (thanks to an extremely active community, a very receptive staff, developers voted for the cause etc.) made RPi kinda... "sexy" also for emulation; i.e. by request of some retrogamers (including myself) the staff with a firmware update provided a 240p video mode on composite video-out, to better (and easily) enjoy 16 bit console emulators on CRT TVs (like the optimus Genesis Plus GX).
Other interesting features are described in the tutorial, provided that, however, the combination RPi+emulation is realistically considered with no taboos but a "light" attitude (eg. playability and not accuracy).
Again, what
@archimage wrote is true, however the equivalence "most recent emulator = less bug" is not always so.
An example is Undercover Cops that in latest release of FBA (dated April 28, 201

has a graphic bug (
which I reported to the author, fixed in next release) while on AdvanceMAME (fork of MAME 106 released in June 18, 2006!) it looks OK. The MAME 106 in fact was a kind of "pleasant interlude" before the sudden change in development which put accuracy (and so, higher hardware demand) above gameplay.
Summing up: to expect a perfect emulation on RPi is, of course, unrealistic. Obviously it will never be comparable to the "real deal" as well as to PC "master race" or FPGAs like MiSTer (even if, paradoxically, arcade emulation today is better on RPi than the latter). BUT to completely disregard RPi is at least a short-sighted attitude since it has several features worth experimenting. Personally I like to consider RPi a kind of arcade "bootleg" open and customizable, surely "gross" but able to offer lots of fun and a graphic experience not too far from the real deal (as evidenced by the pictures) of course without too many worries in terms of accuracy and, above all, at a very low price-tag.
Happy 2019!
