I've used MM, Testors and Humbrol for years, treated them the same (used the same thinners, mixed them the same (heck even mixed them together to get a specific color w/o any probs) and applied them the same way). And never had that many problems that couldn't usually be traced back to operator head space and timing.
The biggest problem I've found with Humbrol occurs after they start ageing (and don't ask me when they start ageing - I've got tins of Humbrol that are 30+ years old and seem to be as good as the day they were manufactured. Yes, they will dry out (which seems to be the problem w/MM/Testors), but that is more of a problem caused by my not cleaning the two mating surface of the tin (which eventually lets air into the tin) than an age problem. I've noticed a couple of different actions in the paint when it does go bad. One is the change in the smell - I'll open a tin I haven't used in a couple of months and it has a really bad smell (I think the paint has soured). In addition to the smell, the paint (which is still liquid) can be applied but as it dries it turns to a much lighter color - and it's in a random pattern, blotches w/a spray gun, streaks w/a brush. The other change is all most as if something has precipitated out of the paint - open a tin, mix it and you notice all kinds of small black spots on the surface of the paint. If you try to apply the paint, you don't get a smooth surface, you get a lumpy one (and the paint will clog up an airbrush). In either case, I simply throw away the offending tin and get a new one.
One last caveat, from what I read, the current Humbrol offering have some problems out of the tin. Apparently, when Humbrol outsourced the production of their paints to China, the quality control of the product went down the tubes, resulting in paints that didn't match from batch to batch, or paints that were extremly slow to dry (or not drying at all). I've been lucky so far (I suspect the paint I purchased recently has been in the supply system for a while), but I also suspect to get bit sooner or later. From what I can understand, it's a rather hit or miss situation, varying from batch to batch, color to color.