4e is pretty survivable & fun at 1st level, especially compared to, well, every other edition of D&D.
GW7, which uses a similar system, is decidedly random, so deadlier than typical 4e - but it's still a lot less deadly than the other 6 (or so, depending on how you feel about Metamorphosis Alpha and the S&SS d20M version)
I ran and played GW quite a bit when it was first out, and the community on the old WotC boards came up with tons of origins and other stuff for it. It is a lot of fun, but the chargen is random so party synergy is not likely. I can recall PCs occasionally dropping in GW games I ran, but I don't think I killed any. Tho, I was running for players who were familiar with or even started with 4e, not players who started with 5e. From what you've said, you probably just need to dial back the challenge a bit until they get used to it. 5e is a poor foundation, so that could take a bit.
One plus of GW is the way it handles weapons, as long as you have one high stat among STR,CON,DEX, or INT, you're set for melee & ranged basic attacks. No one should be sitting out a combat because the enemies are all flying. (There are a couple of origins that use WIS or CHA primary, and if you randomly combine them, you can be left without a viable weapon attack, which is, well, random chargen for ya.)
Missing too much, if not just dice luck, and being frustrated by fliers - I'm thinking maybe they didn't kit themselves out with weapons sufficiently... you essentially get to make up your weapons, so, IDK, maybe they missed that you can have both melee & ranged (and even guns) from the get-go?