The Mumbai local train system, for example, carries 7.5 million people daily.
One possible major issue: maintenance.
I get the impression the local ski resorts spend a good deal of time and materials on gondola maintenance, and it stands to reason that (a) inspection, before anything happens, is much more difficult than a terrestrial (or even subterranean) rail system, and (b) one doesn't have the option of just shunting traffic to neighboring tracks, or even reasonably providing alternate transportation for the current pax, when something does happen.
-Dave
