I didn't use many of the vehicles so I don't know how well they will work out.
Humans who don't have heavy weapons can struggle against elites and brutes.
No-one died in the game I ran so I guess they got by with human weapons.
I made liberal use of luck rolls when winging things like warthogs flipping from explosions and picking routes through a covenant ship.
It worked well for a short game of around 5 sessions, so it hasn't had lots of play testing.
I haven't done the flood yet, I will do them for my next Halo game when I take a break from my RQ/Pathfinder campaign.