I said earlier that I'm running KT120s, but I've two all Partridge Matamps and one Sowter heavy prototype chassis. The 120s draw more power, and it may be that some amps struggle with this, which might cause the 'unreliable' tag?
Certainly mine seem very good so far, one set is twelve months down the line, and all is well.
For 'modern' amplifiers, KT88s would probably be about as high as I would go........