Its 100 years of use in power armor, but its been 200 years of intermittent use powering buildings. Yeah, they're not going to be the most reliable when shoved into something that has a massive drain like power armor or a Gatling laser. MF cells also deplete over time, even if they do last a lot longer than their energy cell predecessors.
As to them being necessary for the Gatling laser... Bethesda really, really hates heavy weapons. The only "big guns" in FO4 are the Minigun, Gatling Laser, Missile Launcher, Fat Man, and Broadsider, and the only one of those that's the least bit useful to use is the missile launcher.