Problem being, I wont know if the abilities are Shadow or something else.
Or are they always pre-fixed with "dark" ?
Oh and probably as important - is there anywhere I'm going to be much weaker?
all the information is out there for you, you just gotta do a little bit of legwork. referencing the link from hotel you can see that the only place you would be taking extra damage as shadow would be from the holy element, although ghost does 5% more damage to shadow vs. if you were just neutral element (h/t to Ren for pointing that out).
basically using shadow armor will not hurt you except when facing monsters that perform holy attacks (e.g. holy light). however you may find it beneficial to use other elemental armors depending on the situation. for example if you're fighting Katherine keyron or mistress, you will need a wind armor or you will get one shot by JT. against drake an aqua armor alone might not even save you vs. his waterball, it hits that hard.