On prices: check with them for bulk prices - I am getting discounts on the P1 with 1000 parts. What price are you looking to get?
UART: They are quite good/active with updating the OS, your friend would do good asking on their partial.io community about the issues with the UART. Btw I use UART functions also and have no issues discovered (that doesn't mean there aren't any).
Bulky firmware: Not sure what you mean by this, the compiled OS or the application firmware? I have created a monitor which samples data at 50kHz, processes this with various transformations and machine learning, has a simple UI, has relay outputs etc. All this application firmware fits comfortably in the Photon or P1 - I have never ran out of code space yet. I could use a bit more RAM for processing but all these small IoT devices do not have much RAM. Not sure what you meant about the SSL/TLS - can you explain, curious to learn what you found.
iCloud dependency: Yes that can be a little annoying if you rather not want to use it. It doesn't sit in your way though so again, not sure what you mean by 'bulky' here. It does work well for on boarding new devices - a service you'd have to create yourself otherwise. Don't forget that the entire OS sources are available (is open source) and you can remove or augment OS code if you like.