Edit Oct-26-2023: added new heroes, adjusted some assessments after more play
Similar to Deft Focus, X-Gene is an efficient resource upgrade that can't be built around because it only pays for cards that come in your hero's identify-specific card set. Among the mutants we have so far, here is how many identity-specific events they have that cost at least 1 resource and a comment on whether or not I would play it in their deck.
Wolverine (10) - Sometimes play. Despite having the most eligible events, there are two mitigating factors. 1 - Wolverine often spends all game in hero form, making this unplayable except on turn one. 2 - He can play one attack event per turn with the claws, so you can't use X-Gene as often as you like. Still such a high count of other events that it's reasonable to play the Gene.
Rogue (10) - Always play. Rogue needs all the help she can get setting up economy
Psylocke (10) - Usually play. I need to test this a bit more, but there's strong competition with The Power of the Mind and using her Knife/Katana-s to pay for things so they flip. 10 is such a high count though.
Storm (9) - Always play. High count and powerful events. No-brainer
Phoenix (8) - Usually play. A bit hard to make use of early, but once you're unleashed, your events are your best cards.
Shadowcat (7) - Usually play
Gambit (7) - Usually play
Cyclops (6) - Usually play. Cyclops decks are often high-synergy and based around allies, so off-theme card slots are slim. That being said, Cyclops goes through his deck really fast, and Phoenix can grab an event back. I like it here.
Cable (5) - Rarely play
Domino (5) - Rarely play
X-23 (5) - Usually play. All 5 are phenomenal cards. With hardly any cards costing 2+, Gene goes up in relative value as the double resource cards go down
Deadpool (5) - Rarely play
Colossus (3) - Rarely play
Even with the low count heroes, it's not ridiculous to play the Gene because if you use it only twice you break even. That being said, the opportunity cost still exists and X-Gene might be less preferable than other resource generators.