In the Vietnam war they used relatively slow prop based planes (OV-!0 Broncos etc.) to identify targets and mark them with smoke for the fast movers. I don't have faith in the Harrier, F35, or any fast mover to have the ability to spot and identify targets anywhere near as capably as a plane flying half their speed, and IMO that's a big part of the CAS role. Sure a fast mover can drop a GPS guided or LGB if the target is lazed by the guys on the ground without even knowing exactly where it is, but that's not always going to be possible. Also slower flying planes give you much more time for targeting your Mavericks so you can turn away before overflying them and also for lining up your guns on that tiny tank down there.
Having said all that, it may be that the G2A threat is so much greater these days that slow is just not survivable anymore, even if you are in a titanium bathtub and can fly home without an engine and most of a wing.
My neighbor was a USMC Bronco-driver, they used them until the mid 90's. COOL airplane.
As far as the 30mm vs. other ordnance; I've never been on the receiving end of air support, but I might take my chances with a 30mm gun run as opposed a 250 or 500lb guided bomb dropped on me!