I agree. The HP Prime is a one of the worst calculators I have ever used.
Some of my dislikes, I suspect, are personal choices. I do not like the touchscreen, I find touchscreens in general to be difficult to use. Fortunately there are keyboard shortcuts for most such operations.
I dislike the fact that connectivity is limited to Windows PCs. The older RPL models would connect to _anything_. I've directly transferred programs between a VAX and my HP48SX.
Those are the personal dislikes. But other problems seem to be more general.
The documentation is atrocious. It's incomplete and it's badly written. IMHO documentation is the most important part of any product. It doesn't matter what features a device has, if you don't know about them, or can't find out how to use them, they might as well not be there. Personally, I also dislike manuals on disk. I find a paper manual much easier to flip through. I think a printed manual should be available, at least as an option.
The Prime crashes far too often. I (and others) have had it lock up doing quite normal things. Like trying to use the spreadsheet having selected RPN entry mode.
The problem I found with the triangle solver (and it's not the only one I've found) is mentioned elsewhere. For another one, try entering 2 of the sides as 1 (say a=1, b=1) and the angle between them as 1e-12 (C=1e-12). It will say there is no solution. There clearly is.
The linear solver is useless. It only handles 2 and 3 variable sets of equations. Heck, I found a commercial Radio Shack program tape for their first pocket computer (aka Sharp PC1211), over 30 years ago, which included an 8 variable solver. That, on a machine with a few K of RAM. Yes, I know the Prime can invert larger matrices, there are ways to solve larger sets of linear equations on the Prime, but...
Then there's the inconsistencies between Function mode and CAS. A trivial one. If you select 'allow complex results', then ASIN(2) gives a sensible result in the Function home screen. But in CAS it gives an error message.
RPN support (and for others here, to me RPL is a very pure form of RPN) is minimal. There should be an RPN-type programming language. Keep PPL if you want, but heck, there must be enough space in the flash ROM to have more than one language. PPL may be more readable than RPN-type languages, but it is also a lot less convenient for one-off hack-type programs. Not all problems get solved by the same tools, after all.
There must be more I've forgotten.
For the moment I am sticking to my RPL machines, HP41's, etc. They work. They only crash if I try low-level things like machine code. They do not give bogus answers.
I was given my Prime. Had I bought it, I would be returning it under the UK Sale of Goods Act. IMHO it is not of 'merchantable quality'