There is a HUGE difference between emulation of x86 for e.g. an ARM Mac to run a legacy x86 Mac binary and 100% emulation of an SNES. In the latter you have to emulate the GPU, I/O etc. and embedded/dedicated gaming devices like that tend to do a lot of crazy stuff and probably little or no documentation other than what has been reverse engineered.
My first computer was an 8 bit Atari, and I can't imagine how difficult that would be to emulate. Sure emulating a 6502 running at 1.7 MHz sounds like the easiest thing in the world, but there was so much more to it. I won't bore people with the details but those who are curious can look it up and see how it generated colors on an NTSC TV, all the programming tricks using horizontal and vertical blanking intervals, the hardware sprites, the goofy sound chip. The worst thing would be the fact that the CPU's execution would be halted for a cycle as each line was sent to the TV to allow the graphics chip to take the memory access slot it would have been able to use (known as "cycle stealing") So not only did you do tricks like turning off the screen during initialization to make it happen more quickly, if you had some code that was cycle counted from the start of a blanking interval (more common in games than you probably think) the emulator would have to account for the exact number of DMA cycles the display stole from the 6502 or it will not be "100% accurate/bug free".
I know nothing about SNES but I'm willing to bet there are all kinds of nasty little details like that buried in it that you have to get right if you want to achieve perfection. Apple doesn't have to do any of that stuff when running an x86 Mac binary in user mode, it just has to substitute ARM instructions for x86 instructions and track the x86 register contents (I know I'm simplifying a bit here) It doesn't care about the difference between the AMD GPU the x86 Mac had and the Apple GPU the M1 Mac had, that's all hidden by the Metal API. So what Rosetta 2 had to do was a walk in the park by comparison to an SNES emulator.
As others have pointed out Rosetta 2 does an amazing job of running x86 Mac binaries, and AFAIK is not "buggy/incomplete" but runs 100% of them and given the uplift in M1 performance versus the x86 Macs being replaced did so at the same/faster speed to boot. The example of the lack of perfect SNES emulators has nothing to do with whether an ARM device can emulate an x86 device perfectly. The only question is performance, and that depends on the starting bar. Apple benefited from Intel's 10nm problems and the fact it was a migration not an additional platform which gave Rosetta 2 a static target that was never getting any faster while the hardware it runs on has.