It felt enormously unfair to unlock the Falling Swords upgrade, only for nothing to happen... until quite a bit later, when I unlocked the ability to get money to drop and actually buy my first sword. Either the Falling Swords upgrade ought to come with one free starter sword, or Falling Swords ought to be locked somewhere behind the coin node. Later on, at least Bouncy Disk was upfront about the upgrade only unlocking the ability to buy the new weapon, not coming with the weapon already.
More generally, progression needs work, particularly in terms of enemy scaling. It's unfun to go directly from having your weapons kill all the enemies in one hit and making piles of loot and killing a boss immediately to losing a full bar of stamina on your first shot while not earning enough resources to even unlock a single upgrade. But that was consistently my experience of starting a new level. It was usually more efficient to just grind earlier levels, but that's comparatively unfun.
Also, it'd be nice if you could click a button to launch the disks instead of only using spacebar. Everything else in the game is controlled with the mouse, why not click-to-shoot?