My experience with them: GBA ones are shit. I bought the 5 main Gen 3 games for $50 on Amazon, what a deal! Turns out the game corrupts after completing the Elite 4 challenge, no matter what. I ended up getting legit carts in a bundle...for $200! I contacted the seller of the pirated games, to his credit he not only refunded me, but he didn't require the games to be sent back, no way these games should be circulated for unfortunate people to buy, so I've since kept them. It's better than giving it back so that the seller could sell them back, granted I wasn't clear to them about the pirated status, I just said they were, and he took my word for it.
On the DS side, I got a Platinum cart that did everything right, except be able to get backed up in a certain dongle. Crap it's hard to remember, I think the USB one that Pokedit sells. Yeah that's it, doesn't recognize the game, I had to back it up using R4I. Late 2015 I learned that this issue meant the cart was pirated, so I went ahead and bought another copy, legit, dongle supports it, hooray, and then not too long after that comes TWLSaveTool, effectively making that dongle obsolete, and really, almost invalidating the second copy's purchase. But who knows what else that sneaky Platinum cart has? I just experienced the save backup issue, I don't know if TWLSaveTool would support it. Both legit and pirated carts have the same save too, so less of a reason to really try it out, as well as being away from the older Pokemon games for so long, needing to keep up with the current happenings.
Basically, buy legit, talk to the seller to ensure they're legit, return and request a refund if they're fake. Have a hard stance against them, mistakes like the one I made can be damaging in one way or another. I was only lucky to redeem myself through buying other copies.