SF6 is my first fighting game ever, and I have complex feelings on this topic.
When I started I intended to use modern controls as an easier way to get into the game. But after an hour or so of playing, I wasn't having fun. Mainly because of auto combo. I got too much for free and didn't feel like I was earning my wins. If it wasn't for auto combo I probably would have stuck with it.
So I switched to classic and am grinding my way up the hard way. Currently in Bronze 4 and still lose a lot of matches to execution errors, dropping a super, messing up the one combo I know haha, etc. So I do find it tilting to lose to modern when I know they don't have to go through the same struggles.
I'm sure it's the case that at higher ranks the difference is minimal, but for new players modern is a huge advantage, and IMO way less fun if you're actually trying to improve at the game.
That said, even in bronze I face very few modern players, so it's not a big deal.
Overall I think it's a fantastic addition for people just having fun. I play with my wife and she has much more fun with modern controls than she would with classic.