Complain, complain! I may have mentioned this before but I have an "issue" with Particle's browser-based program development. I'd give it an "A" for looks but lower marks for the rest.
The worst feature concerns error message feedback when you make a dumb mistake in source input (Sue me! I make mistakes). A recent example: for a 1-letter typo in a function name: 20 lines of error messages, none of which point out even the right line number. I've taken to copying such source to my on-line Raspberry Pi where I try to compile it with GCC; there, I can get a reasonable error message. Errors get an "F".
Another easy user mistake you can make: If you compile source meant for a Photon when the device you selected (usually left over from compiling days before) was a Raspberry Pi (or vice-versa) you get the usual slew of messages, none useful. Grade "D-".
When you have written 20 or so programs, the listing of their names can become an issue. Most recent sketch is always the bottom of the list (oldest to newest). You can't use alphabetical, newest to oldest, or best yet, be able to group them in folders. Grade "C-".