I understand the USMC snipers where taught how to use their old 10X scopes when they needed to engage enemies at close range as well. Doable, but, for me, much tougher a shot to make than with a 6X42 Leupold.
The other drawback could be when using it under low light conditions. With a 40mm scope, one might have trouble seeing the target or especially a fine reticle like a Mil-Dot. E