Are frozen onions any good? (And general advice with frozen veg)
My general rules are twofold -- 


Is the item something that can be frozen whole? (yes for corn kernels, peas, lima beans, etc.) 
Is it something that I'm going to reheat, but not cook over high heat?


So, part of it's the items themselves, but the other part is how I'm preparing them ... and stir-fry just isn't something that I think lends itself to frozen vegetables in my opinion, as you want to cook them quickly, and they're not going to cook quickly from frozen.  Sweating, on the other hand, you might be okay with, I've never tried using frozen onions.

I typically use frozen vegetables for things like pot pies, soups, and such, where the vegetables don't need to be at their absolute prime.  Frozen peas hold up amazingly well; I'll even sprinkle them into paella or other dishes to add texture and color.

My suggestion for your onion problem would be to get larger onions.  You'll spend less crying over 1-2 large onions than 7 or 8 small ones.  At the very least, peel 'em all, then start cutting, so you're not exposed to the fumes slowly over a longer time.