The principle problem with snipers in both vanilla and LW2 is this:
They're all about setting up in a static position and killing enemies as they come. The vast majority of missions are a) on a tight timer and b) require you to play as the active force, rather than the reactive one. Even on the few missions where you are free to set up and wait/draw enemies in, you have to orient everything around the presence of the sniper: you aren't as free to move up into the map, you're much more static once contact is made, and you still have to set up shots for the sniper.
Why? Because snipers can't effective maneuver and fire. If an enemy is in cover they can't just work their way around to a flanking position, they have to try to shoot them out of cover. Which means that either you're taking a lot of low percentage shots or you're using other soldiers' turns to destroy cover or drive enemies out of it... just so you can take a shot that's barely better than rifle damage with barely better than rifle accuracy.
And as the final kick, the attachment to the longrifle means that most of the opportunities for advancement at low levels (grenades, close-range flank shots, &c.) don't mesh with how they play, so snipers tend to lag behind in exp unless you go out of your way to carry them.
Yeah, at high level and high gear they're not bad in their niche, but you have an awful slog to get there and they're still less useful than literally anything else on the vast majority of missions. Most of the time, when I bring a sniper, they spend half their turns trying to move into a position to shoot people, one or two turns missing shots, and the rest of their turns yellow-moving towards evac. :|