I didn't want to turn this thread into a wishlist for the HISE code editor, especially as I wanted to contribute fixes for some of these once I finish this library, but since we went there anyway...
For me the main issue is the low contrast, both just the color palette and lack of differentiation between functions, parameters, variables, control statements, and operators. I realize now looking at screenshots that the whole interface is blue tinted as well.
Sublime Text set to Javascript is quite usable, I haven't really compared with this. The bracket and parenthesis highlighting is far more visible in Sublime, and highlights from either side of the character. In HISE you have to put the cursor after the character, and the highlight is barely visible.
When typing (
, HISE sometimes inserts ()
when it's not helpful to do so. Haven't quite figured out the logic, but I run into it a lot.
Sublime includes autocomplete for structures like, if, if else, for, which I use far more often than the API autocomplete and are huge time savers.
Usually if I'm looking in the API for a function I also need to browse the docs anyway. Sublime also autocompletes functions you have used already, the only thing missing is that tab to next parameter, the highlight state for which I find difficult to distinguish from the usual text selection highlight. This makes me not use it much.
In one of these screenshots, typing appends to the end. In the other, it clears and overwrites.
I use ctrl + left and right arrow a lot to jump through functions. In Sublime one press usually takes you to the next token, in HISE it takes you to the next token or separator, which takes longer and usually isn't what I want.
For keybindings, in Sublime I also have bindings for things like go to next sub-token in a string.
For search, in HISE it defaults to case sensitive every time it's run, and pressing ctrl+f sometimes does not highlight the search field if a search is already open (can't quite track this down atm)
I use code folding quite a lot, which makes it much easier to jump between files without losing my place.
In short, I really just prefer using an external editor, especially as I assume a "reload from file" button would be less work to implement than maintaining a code editor and all the edge cases and user preferences that can go along with that.