It wasn't so much that it was Java (albeit ME without generics) it was more you didn't get an awful lot out the box.
You had to hand roll all your UI with drawing routines that scaled across various devices and handle all the touch/tracker button events yourself.
Networking was a minefield. Depending on your connection (bis/bes/tcp/WiFi) you'd have to pass various undocumented params. Think we had a modified version of some networking code a RIM employee has given us, God knows how you'd have figured it out as an indie dev.
The support and open source ecosystem wasn't really there at the time either. Just a couple of devs who were super prolific and knowledgeable on the dev forums (thanks Peter Strange, if you're about HN)
HSBC app shudder, I rage quit the bank because of it, transferred everything over to Monzo. Truly infuriating and I think it's the only bad app review I've ever left.
First job out of uni was writing device drivers for Motorola Symbian devices on H4 boards (I want to say LDDs and PDDs?). Codewarrior or carbide if you fancied a reboot every 2 mins, 40 minute turn-around times to flash and boot your software, boards without working SD and having to log to the screen ...I do not miss those days at all. BlackBerry was like a bloody dream.