Also his atack isnt "hero" type, its siege as catapults if im not mistaken, wich basically means that he gets more reductions to his damage, also he has a horrible attack animation. He is basically a troll hero with no real use, its almost as playing 4v5 specially during teamfights.
But... Well, looking at the damage type/reduction sheets on the Dota 2 wiki, that means they would do approximately three times as much damage to siege creeps and towers as normal heroes do. They apparently don't outrange towers though, they just match them.
Also, although composite/mixed damage is reduced by both magic resistance and armor, that also means it's increased by debuffs to both... Plus, it goes through magic immunity. Not that you're likely to have someone just wandering around the map popping their BKB before walking up stairs, but hey.
Also, Stworca, I can think of a couple other locations that could be useful mine locations, all depending on the situation. You can deny a lot of popular juke/hook/ambush spots by exploiting the exact qualities that make them popular for those reasons.
EDIT: Also, as far as attack animations go... We've already survived Puck, Lina, Meepo and more... Somehow I doubt that these could be much worse. And if it's last-hitting you're worried about? Remember that you're basically given a couple free CS/denies whenever siege creeps are involved, since you deal proportionally three times as much damage as anyone else can to them. Which isn't bad, for a support.
Plus, I'm assuming these mines trigger on creeps. Which means that when you start getting to be less effective in the later portion of the game, you can still at least bounce around and wipe out creep waves without sacrificing much of your team's fighting capability. Or, heck, leave a present near your base should the enemy try to semi-push/stack a lane while you're off doing other things.