Imo, make sure any player you train an SA on is at least half decent at the skill you want to use: a guy thats crap at FKs will be slightly better than crap with the SA. For other less specific ones they also need to be getting the opportunity to use the SA, if a winger isn't shooting from distance then Shadow Striker won't do anything because an SA only gives an effect when the event to trigger it occurs - no events means no effect - it will not, as I understand it, encourage him to shoot from distance.
I took a not very good CK taker (who sometimes took CKs) and added CK SA (and gave him sole responsibility). He was still a not very good CK taker and his overall ratings suffered. I'd imagine doing more of the stuff he didn't do well had an bigger negative impact. I removed him from CK duties and his ratings recovered (or rather were not negatively impacted).
I have another MC thats very good at CKs and would probably be better with the CK SA but as he only takes them from 1 side I don't consider it necessary.
Just because you have a fast trainer doesn't mean you should train an SA.
Don't try to polish a turd.
PS. As far as I understand it the Nordgens with untrainable SAs are anomalies. A Striker with Aerial Defense will not get many opportunities to benefit from a skill that is triggered by heading in a defensive position - it won't break him though. Other ones can have a use if the get into a situation often enough to utilise it.