Sideloaded ePubs: I have not had any issues, but I have read about freezes from other users on these forums. It is also worth noting that Kobo uses separate software for KePub and ePub books, and some features are not available in the latter (e.g. most reading statistics). Converting to KePub via Calibre will only help in some respects.

KOReader: be sure to read every applicable post before installing Kobo Start Menu and KOReader. The software is very good, but recovery on the H2O is tricky if something goes wrong.

Pocket: only some articles can be accessed on the H2O. (Generally, anything that Pocket presents as an article. Anything that it presents as a webpage will not appear.)

If you are interested in specific bugs/quirks, you may want to browse the bugs thread and firmware specific threads.

That said, I am very happy with Kobo readers.