Going with more than one, two, or three "canon" characters, (to me) is a cheat of a lazy writer. If you're going to do a Trek story, do one in a known era, but create your own characters/ship/station, et al.
I don't mind cameos every so often, in order to branch-off from canon at a known point in TV/Movies into one's own corner of the Trek universe, but there's enough paid commission writers covering ENT, TOS, VOY, to Titan & TNG/TMP at Pocket Books.
For me the whole point of writing Trek is to use the existing characters. Like tie-in fiction, I want to give these characters more stories.
And the fact that a lot of people in this thread have said they find writing their own characters easier rather puts the lie to your supposition that writing existing characters is for lazy writers.
Anyway, back to the original question - so long as I can hear the voice in my head, I can usually write characters to my satisfaction. It's not so much that I avoid the main characters, but I like writing about the characters that haven't had that much chance to shine in the TV stories.
McCoy is the hardest of them all to write. He has a strong voice and character, but his mannerisms and phrases run dangerously close to caricature so it's very difficult to write him and remain convincing (I just hope I'm managing it
