I also got softlocked on the truck stage the same way (boss flying off and getting stuck), playing as Blitzo (I think I used his D,F+A to kill) so I was unable to finish it.
The game is really cool and I'll enjoy running through it with all the characters

.
I think my only complaint is that any enemy/boss with a rapid firing gun (especially the aforementioned boss) feels a little cheap, because they can stunlock you with the bullets and drain a ridiculous amount of HP (over half of a full bar). Since you're almost guaranteed to get hit by all of them, the bullets should do much less damage to accommodate.
EDIT: Hoo boy... after playing a little more, the fun definitely wore off fast. Blitzo is apparently the best character because I had to quit the game with Moxxie and Millie after feeling like I was playing with a handicap. Somehow they seem so much less effective and weaker (Moxie's super move barely tickles the enemies and does almost nothing on bosses). But I think the thing I'm noticing the most is that basic enemies just do way too much damage. In a game like SoR, a basic punch from an enemy doesn't even do 10% of your HP, but in this game, basic attacks from most enemies seem to do like 15-25% of your HP (the flying chick in the first stage, for example). The game also seems to have infinite juggling, meaning if you end up in the air at the edge of the screen, a group of enemies can infinitely juggle you to death. All moves should have a jugglecost (and the characters a jugglepoints) to make it so this can't happen. In general, the game is just a tad too difficult and unfair in certain areas.
EDIT 2: Managed to beat the game (but did get another softlock on the truck stage). Initial thoughts on the playable characters after playing with them more than once each are:
- Blitzo: Perfect
- Moxxie: Underpowered/Ineffective (but is fun to play combo-wise etc.). Super move needs to do more damage or hit more.
- Millie: OK/Fine (Super could use more range, and her shurikan toss is really weak and feels kinda pointless, a cool axe move would've been nicer).
- Coco: Perfect
- Harley: OK/Fine (though I found his D,F+A difficult to hit with at times).
- Luna: Underwhelming (her D,F+A in particular comes out way too slow to be worth using).
Ah, another note: the credits move WAY too slow and cannot be skipped, meaning you're just stuck there for literally several minutes whilst you wait, which means you can't jump straight into another playthrough.