You don't need particularly expensive hardware to get a decent MAME computer setup running most 2D games (on a newer/better version of MAME with low latency such as Shmupmame) at full speed unless you're running the more seriously intensive games (namely anything that's 3D/polygonal stuff). Sure, the hardware's inexpensive, but it's a novelty and isn't intended for a serious setup. not sure if it was just the version of MAME they were using or some other setup issue, but it was definitely a very old version of MAME. Donpachi and Giga Wing both were playable, but Dodonpachi appeared to have a stuttering issue. It's very hit or miss - older stuff like Ms. I know someone local who has a Raspberry Pi and honestly, it's not good - I don't know if it's the versions of MAME available for it or whatnot, but numerous games in the catalogue struggle to run properly, generally shmups made around or after the year 2000.
Let's use a different analogy, it's like eating your dinner with a spanner instead of a knife and fork, you could, but it's not a great idea, and if people started suggesting it you'd likely try to explain that to them. They're only acceptable choices if you have absolutely no standards because, as I mentioned, their featuresets are the exact opposite of what emulation needs, they're the least suitable things for the job yet people keep trying to use them. The Pi you have less of a choice, you're spoon fed already hacked up awful versions, still a bad choice. It also cried at anything with discrete sound, so unless you're willing to make big sacrifices it's an awful choice. Last I used an ATOM it couldn't even run Namco System 2 at full speed without a ton of hacks, that's 80s hardware. Yes, that's an oversimplification, but these are all things the mentioned CPUs are bad at without going into too much complex detail.
so if that code isn't optimally compiled performance suffers a lot. A lot of things in MAME rely heavily on delegates for callbacks etc. There's a lot of context switching, so cache needs to be big enough to keep everything available for quick access, likewise there's a lot of memory being pushed around, bitmaps being updated and copied every frame, so again you ideally want it in cache, or otherwise have fast access to it. Emulators by nature have to run a LOT of code for every instruction they emulate, so instructions per clock is vital. For the ARM based ones like the Pi there's also the fact that some of the most important optimizations in MAME don't work with them (the fastest delegate implementation) so you're losing out there before you even take into consideration the CPU limits. Everything you'd look for in a checklist of 'good CPU for emulation' is missing. Instructions per cycle is ridiculously low, cache is ridiculously low, memory access speeds are low. Maybe I'm biased because I work on MAME, but a lot of my absolute favourite breakthrough moments have come in those 7 years. I know it's not an amazingly popular opinion, and I can only assume that is because on the surface everything 'seems' fine with those old versions, and people have a low bar when it comes to quality, people are only playing the games lightly and don't pick up on the minor emulation issues, or see the games fall apart completely in later levels because at surface level it seems to work. I can understand it when scummy companies do it to sell a product (like that OrangePi in a Genesis style case) because they couldn't give a shit about quality, and just want to sell a crap product and make some cash, but I still don't get why hobbyists push them as if they're going to give a good experience, they're not, nobody should be forced to use something as old as 0.139 just for performance, nevermind 0.78 or older just because the hardware isn't up to it. This is why I don't understand why people keep touting these devices as good for MAME.