I think this game needs more bosses to face. I'll count the sword-wielding ones as just one from a motion sickness perspective because they're too similar.

So, I think that without counting the flying swords, we have seven bosses powerful enough to be considered true bosses.

These are the four that drop the "keys" to open the alien world; there's also the Black Mage; the new one from the sky biome; and finally, the Alien King.