Canonical and their attitude towards the rest of the Linux/FOSS world. This is one good example. I do not want to support such attitude.
Mir (related to the previous point)
No multitasking
Unintuitive design (I am complaining about SailfishOS 2.0, but that's nothing compared to UT's side and bottom swipes).
Alarm doesn't go off when the phone is turned off.
The focus on Web apps and its effect on the native app situation.
Last time I heard, it was nowhere near the functionality completion or optimization of SailfishOS.
After the Ubuntu Edge con and the issue with Ubuntu (Desktop) automatically sending your searches to Amazon for search suggestions, I wouldn't rely on it for my data/privacy