Long days (even 24 hr!) are great for some (or most) greens... maybe even celery, who knows?
Bok choy I've read is the exception, maybe on risk of bolting in long days, since they are so bolt sensitive in general.
I'm not sure about onions and leeks, but never separated mine from other vegs.. My lights are typically left on for 16 hours because I don't have a timer, just turn everything off at bedtime.