The Mosin while utilitarian and reliable has a number of issues that came up in its long military service.
The 91/30 is just plain too long compared to a carbine. I think its more than just a matter of taste in the type combat in WWII, but Russian troops made it work. The rifle is even longer with the bayonet mounted, which is pretty much required to put shots on target.
The safety really isn't very useful on the Mosin, nor is it as safe as the Mauser design which truly blocks the striker.
The bolt release on the Mosin, or lack thereof. Because of the safety issue, troops usually carried the rifle with a round unchambered and with the safety off. When slung up, if the trigger got hung up on anything, the bolt would fall out. Imagine going into battle only to discover you don't have a bolt in your rifle.