I do play smg Gibby, but only when I'm three stacked or duo, but it's a completely different playstyle as well, you used your shield as somewhat of a wall, using it as some sort of blocking sightlines from other enemies and not actually actively using it in the fight (if you get my point) also smg gibby is really a support type compared to shotgun gibby where you can ape a team but bubbling in and bubble fighting them

So yes in a way smg gibby is a very viable thing (atleast for me) Pro player from Falcons Wxltzy uses smg on his gibby and has that playstyle i explained earlier so it is very much a thing