Rover has fencer to deal with raiders. Scorcher is absolutely insane against everything it can hug. It's a bit like complaining about Kodachi not being good against solars.
That said:
Scorcher-micro is really it's own discipline. I am not going to write a tutorial here, but for them to be effective offensively, you need to turn their AI off, maybe even switch their move-state to hold position. Why? Well, let's say you want them to kill a minotaur. Your job is to get them as close as possible, but if you are not super-precise, you might accidentaly click on the tank and thus issue a normal attack-command - which will make them instantly swarm out to keep some distance to the target. It's really annoying. Since the primary job of unit-AI is self-preservation, you can't even say it doesn't do what it is supposed to, but especially with scorchers, it is kind of hilariously anti-synergistic.
Pair this with the mentioned bad matchup against other raiders to get a unit that suffers hard in higher density games. But hey, if you can get a swarm of them into the enemies backline, no other raider will remotely come close to the speed in which that backline is converted to past-tense.
So, do I think they need a buff? I lean towards no. Rover has such a strong early game already (again: fencer) and scorchers are so good in their specific niche that I think it's okay.
About
https://zero-k.info/Battles/Detail/2456592Scorcher-dart spam really was not the play here. Fencers to defend against glaives (and apply pressure against them to push them back), and a few scorchers to attack everything vulnerable. They would have had a field day with that commander at 5:30 or so as Bucky pointed out. But using them to defend against other raiders is really not what they are designed to do.
Also: The main reason why they got owned by those glaives is not primarily that they don't hit them, it's their insane damage fall-off. Glaives can keep distance and apply their full DPS. Bandits are even worse for scorcher in this regard.
