Gunners are not alone in requiring clear line of sight in order to fire. Crossbows do too, their "fire into the air" trick seems to be buggy and hardly kills anything.

Musketeers need pikemen or some sort of tough infantry behind them to deter chargers and give them time to do their work.

Musketeers can skirmish but this is disabled by default. Turning skirmish on bypasses the bugged fire in ranks thing, actually resulting in a higher rate of fire (even though fire in ranks is supposed to speed up the reloading process, in practice the front rank won't fire until the back rank has completed reloading, making it a waste of time).