MG42 is a bit of an oddity amongst MGs; while she has a middling statline (as a 3-star), she has high damage, and the 4th-highest fire rate in the game. What truly sets her apart from the other high-fire rate MGs (Negev, M249 SAW, MG4) is that she is blessed with the far better skill Damage Focus MG, which makes her substantially stronger than any of her contemporaries, and indeed, her new-fangled cousin. While she's not as efficient as the stronger MGs, the fact that she ladles out an insane amount of ammo in a very short time, paired with her access to an excellent skill, makes her outstanding at ripping apart bosses and armored opposition, especially with the proper upgrades. Her main drawbacks are that she takes a while to kick in her skill and that her fire rate is so high that she'll spend a lot of time reloading.
MG42's biggest downside is her grid. While she gives an incredibly strong buff (22%, equal to M2HB's), it's diagonal only and only really works with it as part of a 2MG/2SG echelon.
This also means that, unless specifically in a 2MG echelon, she's almost invariably better as a one-off addition to an echelon rather than being specifically there to shore up a squad alongside a Shotgun, which is somewhat unique. While this isn't a particularly necessary advantage for late-game echelons, it's a godsend early on and she can more than earn her pay in the early stages of the game, with her performance being high enough to justify her use in budget echelons.