Actually in the US, the term "Watts RMS" specifies that the measurement is done according to the FTC Amplifier Rule. Now that rule has its own pro's and con's of course, but at least it's pretty definite. With all of these things, there's a corresponding European rule as well.

Whether the measurement method is appropriate to your use of an amplifier is of course anybody's guess.

Considering that the load impedance of audio amplifiers isn't always the same, it'd be a bad idea to measure output voltage in terms of gain compression points/clipping/max continuous.

I agree the term "RMS" is kinda miss-appropriated, but it's short and tends to understood as something reasonably close to what it's actually about.