A lot of games like this really lack for indirect fire weapons. Mostly your options are grenade launchers, and hand thrown grenades.
I had a situation the other day on one of the Mediterranean maps, where I had a Civilian duck through the only passable hole in the wall, and the last alien on the map pop up behind him. Unfortunately, it was sufficiently early that I didn't even have my first bolter, so I had no way to penetrate the wall. And the grenade launcher doesn't have the high angle it needs to drop behind an obstacle. I finally managed to finish it off with a grenade toss up and over the wall, after getting shot because that was the only place I could throw safely to have it land behind the alien and kill it and not the civilian.
Balance is an issue, but it always is. Consider the Rocket Launcher. It's very useful, and capable of instant killing targets. However, it has to be reloaded after each shot, and it's got a huge explosion radius so it's not a weapon for shooting close to friendlies. The Grenade Launcher is similar, but the arc of the shot makes it useful for entirely different tasks.
I've never been a big fan of destructible environments. It's cool and all, but it ends up making extra work for the designers, without adding a huge amount. Besides, that's not what mortars do. Mortars provide immediate ordinance to individual units. 6 soldiers are dangerous, 6 soldiers and a mortar are VERY dangerous.
I'm not sure what role they have in mind for UGV's. It's an interesting idea, but it was a good one in X-Com and they seemed to blow up rather a lot. That said, a mortar UGV is not at all far fetched.
