re: Genesis Hydra
Being an X-spell certainly encourages a play pattern of just sitting until you have MAXIMUM MANA but it isn't really necessary. Given that you have kept the overall curve of your cube/deck low, X=3-4 often yields some good hits.
I like that it's a green card that plays well with green themes (ramp, permanents mattering, +1/+1 counters, etc.) while not playing particularly well with themes from other colors that normally would be all over a powerful "ETB" effect (blink, reanimation, clones). It punishes bounce somewhat (like Thragtusk) without making it worthless (like Thragtusk). Scales with mana, plays well with knowing the top card(s) of your library, plays well with your own bounce effects or "symmetrical" bounce, etc.
Also an X-spell that doesn't win the game on the spot with infinite mana which is kinda nice.
re: demon dude
Pros:
Can Pod a 6/6 demon into a 2/2 that makes a 5/5 demon. Then you can Pod the 2/2 into... another 6/6 demon. Spooky.
Human (and Cleric!!) tribal, BB for devotion, 2/2 for 'lark/Alesha, good blink/reanimation/etc. target, etc.
Neither half dies to Doom Blade.
Good against a lot of green cards and red cards.
Cons:
You lose 10 life before it can get through a Lingering Souls. Spooky.
Bad against counterspells (5 mana), bounce (good body is the token, downside is on the main body), Pacifism effects, Stifle/Trickbind/etc., dragons, spirit tokens, faerie tokens, tappers, etc.