I just bought a cheap 1/4-wavelength, 2m. mag-mount antenna. The package says it can handle 50 watts. I can think of three things that could limit the capacity:
$I^2R$ losses in the connection to the feedpoint and in the antenna wire itself: increases with increasing current, produces gradual breakdown of materials;
dielectric heating at the feedpoint: increases with increasing voltage and frequency, produces gradual breakdown of materials;
dielectric breakdown at the feedpoint, happens suddenly when voltage is too large, produces immediate failure.
Are these right? Are there others?