As you may know, traded Pokémon gain experience faster than other Pokémon. However, (I think) they don't gain more EVs from beating other Pokémon. This means it takes longer for them to reach the EV cap and therefore they can be higher-leveled yet have poor stats because of not enough EVs.
Where did you read that?
Anyway, since he was asking about global trades and not in-game, I can tell you right now that that's not true, otherwise Trade Corner would suck. I'd be doing RNG requests for no reason, cause people go and EV train the Pokemon I trade them. So yes, global trade Pokemon can still gain EVs as long as there's room for them. I imagine in-game trades would be the same. idk where you read that they don't get EVs. That doesn't make sense to me at all. I'm really hoping your info on that is wrong, because that would really suck. I use the in-game trade Petilil in Black and Black 2 a lot, so lol.