My F-100 2x12 combo has performed flawlessly for me over 4 years of practices and gigs. I've only had to retube the power section twice so far and I had one microphonic preamp tube.
Off the top of my head, the problems that I've seen are few and far between, but here are the more serious ones reported in the F-series Lounge on HC:
1) Blown Screen grid resistor. This happens sometimes when a power tube blows. It's an easy fix, but can be difficult to diagnose. Normally, one tube will be glowing red hot.
2) Screen grid resistor broken off solder points. I don't even know how this could happen, but it did. The actual resistor was fine and the owner simply resoldered it back in. Everything worked fine after that.
3) Mesa Custom 90 speaker with broken voice coil. This is another strange one, because I've never seen a speaker fail this way. From what I gather, the speaker's internal voice coil connections broke.
4) The first batch of F-30s had an incorrectly rated capacitor on the power supply, which was replaced by Mesa on warranty.
5) Generally, most of the problems with the F-series are simply due to being a tube amp. Preamp tubes go microphonic or fail and can cause an excess of noise. Power tubes can lose power output or make the volume fluctuate randomly, and even take a screen grid resistor with them when they blow. That's why you should try to take note of how the amp sounds so that when the power tubes start to lose their luster you can replace them before they blow and potentially cause bigger problems.
As is normally the case, when a person has a good experience with an amp, they'll tell a few people about it. When a person has a bad experience with an amp, they'll tell everyone they know.
