I definitely want to see Grillers nerfed in Splatoon 3 if not sooner. Between their single point of vulnerability, their speed, and the smallfries, they're a threat every which way. I think that if they dropped just a few smallfries to keep everyone on their toes, moved just a bit slower, had a longer delay between splatting their designated victim and heading for a new one, and had a wider soft spot, they'd be balanced with a normal round.
Glowflies only bother me if I have multiple incompetent teammates, or if I have one incompetent teammate and one with a weapon that's just not great in the mode. But most weapons do just fine. Rollers annihilate them; even the carbon roller just gets knocked back but kills them anyway. Blasters and sloshers kill them en masse, although the Rapid Pro isn't really efficient enough. Most chargers slice through a whole line of them like butter (not sure about the Bamboozler or Grizzco). Most shooters and dualies are fine; the exceptions are the weak ones (Jet Squelcher in particular) and the ink-hungry ones (Pro, .52, and especially the .96).
My experience is that if one person goes down in a Glowflies round, it's most often salvageable without using a special. The only danger is the chain reaction, where someone instantly gets the Glowflies who wasn't prepared for them. Even then, any special but the Stingray can salvage things (and even it sometimes). In Grillers, one death is a big problem every time, and more importantly, it's hard to even use specials to revive them. Grillers are tall enough to reach Inkjets, they get in the way of Bomb Launchers, and they aren't killed by Splashdown (and, like with Inkjets, can even kill you at the peak of your jump).