I would be interested in an in-depth outline/timeline of all these bands. It's quite confusing.
Well, first off, Van Halen and Van Hagar are the same band. David Lee Roth was the original lead singer from 1974 to 1985, and then he either quit or was fired, depending on who you listen to, and was replaced with Sammy Hagar, who fronted the band from 1986 to 1996. There are a lot of VH purists - like Laz, I guess - who speak derogatorily about "Van Hagar", aka the band when they were fronted by Hagar. I guess because the band delved more into power ballad territory at the same time(where they'd been more straight ahead rock before with Roth).
But straight up, David Lee Roth is a better entertainer, a better "frontman", but in terms of pure vocal ability, Hagar is the better singer, and it's really not close imo. I'll die on that hill.
Either way, you can't call VH with Hagar not "legit" just because you don't care for them.
Aerosmith put out their first album in 1971 and became pretty big in the 70s, but by the early 80s they were falling apart, Joe Perry(the lead guitarist and second most famous person in the band after Steven Tyler) briefly quit the band, and Tyler went to rehab(I think others did too). Long story short, they regrouped in the mid-80s and started letting outside songwriters help them write and they made a HUGE commercial comeback in the second half of the 80s and 90s - just a huge string of hits.
Again, I disagree with Laz. Aerosmith's 80s/90s material is certainly different from their 70s material in that's more pop-rock than rock, but I think it's enjoyable pop-rock. Nothing earth-shattering and nothing too deep, but enjoyable.
Guns'N'Roses formed in the mid-80s and put out their first album, Appetite For Destruction, in 1987, and it was enormously successful. Laz is saying that that album picked up where Aerosmith's 70s output left off, before they went pop-rock. Not entirely sure about that - 70s Aerosmith was more blues-based hard rock, while GnR was a little more metal, imo.