it depends on the font vendor, whether they introduced a proper weight map and metrics
imo in case of custom loaded webfonts it’s more reliable to explicitly set the font-family you want. i had issues, for example, with Geometria before, when relying solely on ‘font-weight’ and setting it in @font-face really fucked up letters on safari. seems, it didn’t care about my declarations, just got ‘regular’ and applied system built-in pseudo boldness, which looks ugly.
tho, of course, safari is a bitch