What cards would benefit to such a huge extent from this energy gain, to a point where it's broken?
Did you ever face
Scramble Energy in tournament play when it was legal? This is not an all or nothing point to the argument, I just honestly wonder and apologize since it is very hard to answer this not knowing.
I tend to be long winded, and there is much to cover. The shorthand version is you're supposed to require effort to get a Stage 1/2 Pokemon with three Energy into play. Yeah, a lot of cards currently make that too easy but this just makes those cards look like slackers!
Plus it isn't like something with a good attack for (CCC), but any attack on an Evolution for three (or less) Energy. Even a minimally competent opponent is then able to take down a healthy attacker you spent time building with a single shot, buying time for them to either Bench whatever they
Scrambled (saving it for later) or build it manually into a big attacker (while you were set back to square one).
Competent players will exploit the card. Before specific Pokemon combos are advanced but deck generic combos, like giving up an opening Pokemon so you can use
Twins to... you guessed it, get
Scramble Energy and
Rare Candy into hand. Then there is learning to pick your targets... often the player using
Scramble Energy won't be worried about your current attacker, but whatever is most vital. If you take out what they
Scrambled the next turn, you're ahead a Prize again, and they can repeat until you have a false lead of one Prize, with nothing good left in play while your opponent is riding that
Scramble Energy.
As for specific abuse, I already mentioned what
Electrode can do. Think about it: hitting a single
Scamble Energy probably means its effect is activated, and any other Energy attachments are gravy. Extra
Scramble Energy just require viable targets, since they won't go away when attached to Evolutions, even if they are only providing :colorless: at the time. It takes only moderate skill to wear you down with that kind of set-up.
Drifblim would be utterly broken and might rise to staple status. Any reasonably aggressive deck would just spam
Drifblim and
Scramble Energy (possibly a few copies, due to how this works) to constantly obliterate the opposition's set up once said opponent got ahead a single Prize.
It feels less cheap when a deck built around exploiting the "come from behind" cards does just that: I consider it a legitimate strategy myself. Most such cards work in several decks, and it is quite vexing to lose because an opponent brought a Stage 2 Pokemon out that was naked (or even was a naked Basic Pokemon) the turn before and suddenly its a Stage 2 with three of any and all Energy attached, able to use a great attack (possibly its best). Before they do that, they use
Pokemon Catcher to force up something totally healthy up from the Bench to OHKO it, and before actually attacking
Rocket's Admin... sorry
N shrinks your hand to almost nothing. Now your opponent just needs something that can finish off your almost KOed attacker, and the only benefit of your field is that you won't lose immediately due to having no Pokemon in play.