It depends on how long they can sustain this. If the magic is based only on concentration, and they can therefore go on for as long as they want, it'd be fantastic, as it wouldn't require any ammunition.
A big advantage I'd see in a war with magic users basically being able to mimic the effects of a machine gun or larger weapon, is that it is far, far more easy to transport. Being able to field a large number of these magic users, could be used for interesting blitzkrieg (lightning war) and guerilla warfare tactics, and thus the army with the bigger stick, so to speak, would have a big advantage in mobility. I imagine something like how in Brandon Sanderson's Stormlight Archive, wars are fought just to get the legendary shardblades.
So yes, I'd say that it would be effective in modern combat. Especially in rough terrain, not having to carry a gun with you would be very convenient.