I've user phpbb for 5 years now... can't say I'm fond of it. Well compared to mybb and vbulletin which I also used, I kinda like it, but in general I'm getting less and less happy with it.
I'll say that a lot is about the looks: even with a nice-ish custom made theme, the problem is the template, which I edit a little bit but can't do anything massive, as the next update will likely break it. Even if responsive, as quite a few themes are by now, buttons are usually small (they have to be as there are so many of them) and they just feel old.
Also, the practical aspect of it is difficult: as a mod even just deleting a topic is a matter of opening the mod panel, choosing the topic, deleting... while other software just have inline tools for that.
As a user, bbcode is unfriendly, and the quick reply doesn't have it. There is no automatic resizing of the profile pics (users have to resize them beforehand), and
Administering it is also well, not pleasant. It does the job, but when I have to get into the admin area I'm never super happy.
There are a lot of extensions, it's true. And they break at mostly every single update, which is why I stopped using anything outside the officially supported extensions. By the devs words, they are very difficult to maintain.
I'm considering migrating to flarum too, it feels so refreshing!