They are kinda tame compared to stompas weapon load-out wise. The Melta blast is basically the same as the Cestus Assault Ram, only less mobile. The D weapon is CC - so that's not really that bad with the 3 attacks in CC. I can see these things getting bogged down in CC pretty easily. What are the rules for Stomp again?
6 HP is going to be Melta Sternguard bait.
I'd like to see one in action for sure before I make any judgement calls. But really I don't think 1 will be that bad. Now if someone wants to sink the $$$ to play 2-3 in an 1850 game, well...Have fun with that.