The most recent rules for the Big Mek Stompa are in the apocalypse warzone Armageddon rules in the most recent apocalypse book.
[That's this one]
They are complete and supersede the older forgeworld rules. You don't have to worry about the mishap - it no longer has that rule.
It's not a great superheavy as you need to roll to hit with your main weapons on your fairly shoddy ballistic skill, but nevertheless if you do hit the weapons are pretty nasty and you can always chop stuff up with your mega-choppa.