For novels I want and prefer epubs, but also non-novels if they were released in the last 5 years or so. PDF isn't magic, and there are bad pdfs out there too, scans of photo-copied books and other nonsense.
Are you speaking just about technical books?
Because I can’t imagine anyone trying to read a novel in epub vs pdf on a phone or epub reader and going with the latter.
As for phones, screens nowadays are almost the same size as readers and with more resolution. E-ink is more comfortable for longer sessions, but if you find such a size unusable you might just have poor eyesight.
I have read many hundreds of books this way.
The problem with a tablet is that most tablets, especially the sort that are good for seeing entire as-printed pages at once, are too big for me to keep the entire screen in focus without wearing glasses. (with that said, foldables improve things here, since the aspect ratio bottleneck is typically width so being able to double the width on the fly makes such things more readable.
Only benefit is browsers have built-in support for the format.
Although for cases like Kavita, storing in the file would be problematic if multiple users want their own annotations without concerns of data leaking.
PDFs have pretty excellent support for metadata. If the collection software doesn't support at least Dublin Core, that may be kind of their own fault...
I'm sure Adobe Acrobat also supports, but that's not used in the scene.