I think it can at times be both. A lot of guys get subbed by leg locks because they don't know how to defend them but mostly that is when guys who aren't leg lock specialist win with them then it usually shows weak submission defense.
Shogun vs Randleman and Lesnar vs Mir are perfect examples.
However fights like Aoki vs Alvarez, Brown vs Imanari, Condit vs Kitaoka are guys with a different level of leg locks then just your average kneebars, and heel hooks beating guys with great submission defense.
Aoki would do fine in the UFC. Guys like JZ and Alvarez are as well rounded as every LW in the UFC especially JZ.