- It inserts "ads" for other Reddit posts in the middle of a comment section. If you're not paying very close attention, it's easy to miss that you're literally not seeing all the comments on a post, because you have to scroll down past the "recommended reading" or whatever they call it and expand the comments below the fold.
- it often makes it impossible to see NSFW content on mobile websites without using the app, which is annoying if you click through from a Google search result on your phone. This applies to text posts too, which is all the more annoying because despite the name, the NSFW tag is used to tag more than just porn (hide spoilers for TV shows, punchlines of jokes, etc.)
With that said, I don't think old.reddit is that good either unless you only want to read text-only posts, and it's pretty much unusable on mobile.
Old Reddit is nearly unusable on mobile, but it's still better than the new Reddit on mobile, which is... actually quite impressive (and not in a good way).