Avids get a lot of bad press for unreliability. In my experience, the main problem with them is their plastic pistons. The constant exposure a rear MTB brake has in the UK to shit, probably isn’t ideal for plastic pistons to cope with and remain free-moving in the caliper body (as ever, US-designed components not up to the job in UK weather).
I bought a piston replacement kit and since fitting it, this problem has gone away. It’ll probably come back again in 12 months’ or so, but by then I hope I’ll have sacked them off for some of the new, well-rated Shimanos.
What I haven’t fixed yet is the constant vibration from the rear brake under mild modulation. My latest theory is that bedding in new pads is putting too smooth a surface on the pads, so I’m trying out fresh organic pads without bedding them in. If that fails, I’ll try baking the pads.
Avids work great when they’re working great but nowadays they’ve got to really step up to compete with the boat-load of very good alternatives.