I was talking about stinger with
@ClarenceMage just yesterday.
He might be able to contribute.
I was trying it out when I was dicking around with Team Pali, specifically as a way to boost Parasoul neutral game.
What I found was that M Stinger was ridiculously versatile at every range.
Full screen- Call assist, do L shot. Dash in, and you're following in two fireballs. If your opponent jumps, swat them with j.HP, if they block, they fall down and get to block the M Stinger setting up mixups. If they get hit by the shot, then L shot will combo into the stinger, and you can convert into full combo.
Mid screen- j.HP + Stinger to cover multiple angles, M Shot + Stinger to cover the angles. Stinger + L shot to Harass, all pretty safe.
Close - Close up pressure game is boosted greatly by Stinger, by doing blockstring into Stinger, L shot, you dash up while they have to deal with stinger. It creates more situations that are unavoidable by pushblocking. Because of how long it travels and its long startup time, it can come out when you're in the middle of a special, leaving you safe after many things that are normally punishable.
Incoming Mixup - Incoming mixups with a slow traveling fireball are great. Go back, call assist, then super jump j.MK hold and hover left-right for an unseeable left-right with the fireball coming out.
That's what I remember, but there were heaps of small situational uses that I made up on the spot. I haven't explored it with other characters, but I found it worked really well with Parasoul.
This is why I rate it higher than c.MP assist - It allows characters to play the game at more ranges, and if you are able to be a threat at more ranges then matchups will probably lean more to your favour. c.MP is great as a lockdown assist, but that requires you to already be close to your opponent, and having an assist dedicated to PURELY close-up lockdown is redundant when every single character has fantastic pressure/mixup game when point blank.
Edit: Come to think of it, dedicated lockdown assists aren't redundant if they're SUPER RIDICULOUS like Sekhmet MUDAMUDAMUDA. However, Painwheel's c.MP does not lock down nearly that hard to be a worthy pick.