Amazing month for PS4, weak as fuck for Vita.
I've been playing Sky Force and the progression system sucks. You can tell it was a F2P game converted to premium because it feels like a grind without the IAP.
Beautiful game, fun game, but awful progression. You collect stars in levels that you spend to upgrade your ship, but initial new tools cost an assload of stars and further upgrades barely make a difference.
What's worse is that you need to achieve a certain amount of Medals to unlock the next stage, beating it isn't enough. There's 4 requirements for the 4 Medals you can earn and they are always the same: destroy 70% of enemy forces (easy) rescue all survivors (fairly easy) destroy 100% of enemy forces (extremely obnoxious) stay untouched (hard).
What all this amounts to is replaying the same handful of stages over and over and over again and it is really boring. You'll need to grind for stars to power up and make achieving some of those Medals manageable and you'll need to replay levels whenever you fail because a fighter got away or you took a hit or missed a guy.