1/4" to 1" gaps in corners of my newly "professionally" installed drywall
It might look sloppy now, but if they are conscientious when taping and finishing, that will not be a problem in the least. It really would only take one quick setting-type coat before taping to get even a 1 inch gap as ready for tape as any other corner. (Assuming there is blocking behind it to back up the compound - there should be, otherwise what is the drywall attached to?). It's important though that the corner tape reaches the drywall - if it doesn't, there needs to be an additional run of tape over the filled-in gap. But other than that slight setback, the finished corner will be as nice as any other corner.