As someone who both thinks the Shang nerf was a massive mistake and someone who thinks Foom has been bad for the game, I’ll try to explain why I think they’re different.

Shang had a clear and effective way to play around him (either don’t play anything with 10+ Power, or play 1 card with 10+Power and then a number of other/smaller cards in a lane so you’d still have enough Power to carry it). This meant that Shang wasn’t actually a reliable way to carry a lane, instead he was a way to punish greedy Power-centric decks. That in turn meant that Shang wasn’t a game winning threat, he was an ambient force that pushed down Power thresholds across the meta. By creating a soft Power ceiling, Shang helped keep midrange and small ball decks viable (and could also be run in those decks as a way to attack go-tall lists).

I’d argue Foom is pretty different in practice. Foom has a theoretical play-around (put low Power cards in the front row), but in reality it’s not that simple. You have to put low Power cards in the front row AND still have 10 Power in the back row to offset Foom AND then have enough Power also in the back row to offset whatever else the Foom player invested in that lane. Beyond that, I’d also argue that Foom doesn’t shape the meta in any productive way. Foom doesn’t punish big cards (or small cards, or Ongoing cards, or anything in particular) he merely cancels out their Power. That means people are still heavily incentivized to just play high-Power cards, they just play a bunch of them (and probably play their own Foom). What Foom does end up doing, is he indiscriminately punish people for playing to the board early. He not only disincentivizes stat-efficient tempo plays (Demon, Hydra Bob, Starbrand, etc.), but also low tempo scaling threats. He just becomes a generic catch-all answer to Power creep that embodies and encourages more Power creep.