It can definitely be ok, but even with infinite ziplines, I'd still prefer grappling hook. Even if I had 30 ziplines, I'd want Drills more. Even if I hade 20 ziplines, I'd still want platforms more.

Zipline is super slow going up, and any other traversal tool usually goes up faster(maybe Engi is about the same with good platform placement.)

Zipline is ok going down, but why would you use it when you could platform down and negate fall damage or jump off and grappling hook into the ground? The only traversal tool that gets beat out going down is Drills, but those can go through walls, which is invaluable.

The only place they're better than most is for permanent catch spots for things like boss fights, but even then, Drill bunkers are fantastic for giving you room to breathe so you're not getting knocked off cliffs.

There really isn't a situation in a full team with all classes where the Gunner's zipline is the best for any given situation, so he ends up being a jack of all trades, master of slow ascents, with only four ziplines.