You can absolutely get behind cover for the sniper...but in the heat of the moment, it's easy to forget server latency. From our end, it can look like it shot us through a wall, but on their end, it still had line of sight.
The jetpackers can sting, but you can dodge out of the line of fire, then LoS them. The whole point seems to be to avoid being shot by them. This is further evidenced by the fact that a large number of things will knock them down to the ground, and that they only fire for a bit, only when in the air. They make up for this easy interruptability, and long gap between shooting by being extremely deadly. They're really rough on hard, but a nice kind of rough. They stop you from letting the flank you, as that's a death sentence. Much like letting snipers sit back and plink you, or flamers close in on you are also deaths.
