There's nothing wrong it @ 3/8", but you are losing some power due to the reduced mass of the anvil and sockets. [EDIT - the guys at the Torque Test Channel appear to have determined that difference isn't much - I do value the broader range of larger sockets available on 1/2", including some specialty sockets.]
I have a 1/2" Stubby and 1/2" Mid Gen2. They're a fantastic duo. I spent today dropping the subframe on my Mini, which is unfortunately required to replace lower control arms, and getting started on replacing the struts/links/etc. My Stubby did 80% of the work, but there were a couple spots it wouldn't even fit and required the use of a wobble that robbed enough power that the Stubby couldn't manage. There were also a few big bolts that were just too much. Out came the Mid and we carried on.
The Stubby isn't an "everything" cool, but boy did I appreciate its size and weight today. If the Mid was my only choice, it would have definitely been a harder job. If it had been a 3/8", I'm pretty sure the Mid would have come into play much more often - probably a 60/40 split. But that's also one of the more challenging days, too.
If all you have is 3/8" Impact sockets, then it is what it is. I chose 1/2" with the intent of only having 1 set of impacts, for the extra grump it provides and for the upper size range for things like axle nuts. My Sunex set starts at 9mm, which is fine with me - I'd think twice before smacking anything 8mm or smaller with an impact wrench - even on gentler settings.
If your Mid is 1/2", I'd really recommend a 1/2" Stubby.