They are all rather well written. But if your like me you'll notice inconsistencies between books, like in the number of shots it takes to kill off an Elite. Nothing overly important but yeah. i enjoyed the books and have just started reading Contact Harvest.
As for a reason for why they changed the authors well they might not have been able to hold on to the same author for every book that was planned. Or maybe they had several people writing the books and chose the ones they thought were best?