I suppose it's technically possible to edit levels with a hex editor, but I have no idea why you would do that. How would you expect that to work?
what makes you think it doesn't work?, FCEUX's hex editor is quite handy and has some neat features, like the code date logger highlights what parts are data and what is code, that way I figured out Solstice's level data. Plus it allows to edit game real-time(usefulness depends on how the game loads the data, in my case Solstice reloads everything when I exit and re-enter room), undo edits(even if the game crashes I can re-load save state or reset game and I dont lose any changes, and I can undo changes too and everything goes back to normal), show tbl/table files, and save the rom file so at least for me it was really, really handy and useful. my only complain is that there is no option to make font larger or change the highlight colors.