On android mobile phones the ad at the bottom of the screen is so large (about 20%) it interferes with scrolling motion, reading, covers commands, etc.
Every time you change to a new page it reappears (which on a forum is quite a lot), and it is difficult to close without accidentally opening up the advert. Were they time dependent rather than page refresh dependent it would be less of an issue.
You are left with the option of trying to close it multiple times a minute (and then closing the inevitable redirections to the advert) or a significantly reduced browsing experience.
I had to install an adblocker on my PC (which I don't really want to do) because I find the adverts insufferably intrusive to the point I didn't want to use the site. I've not found an effective way to block them on my mobile and as a result I spend much less time than before on the mobile site.
I don't have a problem with adverts, just these adverts. The old adverts were fine, visible but unobtrusive. Honestly, RF is probably the only 'non-dodgy' site I can think of the the (non-video) adverts cause me any annoyance.
While some people don't seem that bothered, there are others who are genuinely being driven away from the site or at least being significantly annoyed by it. Almost all other sites manage to find a much better balance, including RF until recently.