I'm a bit contrarian in this, I guess. For raw aim and pattern recognition in very dense maps/EZ, I think it can be helpful, if you're playing things with relax that you otherwise couldn't play at all. Of course it's not the same as playing normally, and if you only play relax for too long, it'll take a few weeks to adjust back to playing normally. But I don't think it's harmful, the worst thing for your aim, etc. Just don't confuse your relax skill for your vanilla skill, they're separate, and don't play it too much. And if you're going for improving specifically relax scores, consider 100s as misses, as they suggest an aim problem. I think tapping at the same time is probably a bad idea, though. You'll get feedback suggesting you're tapping on-time even when you're not, it could mess up your rhythm sense and I think it counteracts any positives you'd get out of playing difficult maps with relax.
I played a lot of relax in my first year playing more seriously, and I still play it occasionally. I feel it helped me to find difficult/"legendary" maps less intimidating when I could finally try them for real, helped my reading, and helped me get physically used to higher spacing.
I feel when people think of relax, they think of a scenario like someone practicing a PP farm map with relax thinking they'll get it in vanilla if they practice it enough that way. They're right that in that scenario, it isn't going to help you much. But if you take the opportunity to use relax to get used to legendary, weird, funky, and fun maps, to experiment with mods, and just have fun playing things you can't otherwise play, I think there's benefit.
If you jump up a few SR from what you can normally FC (when I was FCing 3* and 4*, I played relax on 6* and 7*), none of the relax "bad habits" like round aim and aiming too quickly really transfer over if you're playing it in moderation. With relax, you'll be able to focus fully on how to interpret the more complex patterns, read them faster if necessary, and how to comfortably get your cursor from point A to point B, which helps in your vanilla play.
(Sorry for all the edits, ADHD)