There are a bunch of different versions with different capsules. For example, the Modmic Uni doesn't sound very good, but since it's unidirectional (it's a 6mm cardioid electret I think) it is rather more resistant to ambient noise. The Omni has your usual run-of-the-mill 6mm capsule, these are all very similar in terms of sound and noise performance. The Uni is kinda good enough for pure communication, but you'd really wouldn't want to use it for content production.

Also, being electret capsules directly wired up to your soundcard, the soundcard has quite an influence on the quality of the audio (mostly in terms of noise and hiss). Meanwhile the digital versions don't suffer from bad microphone inputs.

It depends a lot on your sound card I guess. Pro streamers use them on twitch as portable options (like Seagull) and they sound great to me.

The only real downside to it is the cable is sort of flimsy and the 3.5mm termination is not great quality. That's how my last ModMic perished, although it lasted a few years.

I got their wireless one recently and everyone I regularly use it to talk to immediately noticed the quality and commented on it. Can't speak to the wired ones.