I haven't used Youtube Premium myself, but I've heard many reports of both ads and promotional crap that gets through to premium users.
I'm also pretty sure that data collection is still on, meaning that you'll get tracked and ads elsewhere (such as on Google.com) based on tracking your youtube premium behavior.
I've had Youtube Premium for as long as I can remember. I subscribed to Google Play Music, and they had a promotion for free Youtube Red with a subscription to GPM. I've kept it continuously since then, for free, and I've never seen an ad. I forget that Youtube even has ads until someone else puts a video on. I've also watched a lot of Youtube. I've seen every RedLetter Media video at least twice. This is all anecdotal of course. Maybe I've just gotten lucky.
I have the same experience. I watch lots of educational YouTube content and share my premium subscription with my wife and kids. I could never go back to ad-based YouTube.
Try it, is free to try. The experience is quite great without ads but do not trust me or others, try for yourself if you watch YouTube and want to support creators but hate the ads.
I'm also pretty sure that data collection is still on, meaning that you'll get tracked and ads elsewhere (such as on Google.com) based on tracking your youtube premium behavior.