I'm just going to list them.
1. The envelope UI is TERRIBLE. Everything is incredibly unclear.
2. The wavtool is incredibly choppy and makes even a VB as bulletproof as Yamine Renri sound absolutely choppy. I can't change my wavtool to something GOOD like wavtool4vcv.
3. There's nowhere near as much granular control
4. The phonemizers, especially the ones for English and French, are of incredibly poor quality, and often produce far worse results than manual note entry in og UTAU
5. So. Many. Goddamn. Crashes. I've had UTAU crash on me maybe twice in my 2 and a half years of using it, and I've had OpenUTAU crash at least 50 times, including one time where it managed to crash Windows.
6. It's much slower than og UTAU to render, if I'm using my preferred resampler (Moresampler)
7. It makes VBs with extras a pain in the butt to use due to the phonemizers not playing nice with extras.
8. Most plugins do not work.
9. Getting things to work like a character portrait is really annoying.
10. Save last play is a feature I use a LOT that is not available in OpenUTAU
11. Trying to get a project to play brings my PC to a screeching halt
12. The dictionaries being used are just BAD, and you can't customize them.
EDIT:
13. There is absolutely zero reason this program should take longer to launch than it takes me to open a UST in the original version of UTAU, pick a voicebank, and tune a 30 second long demo piece using the pitedit plugin