I saw that thread title. Made me chuckle.
The interesting thing is that you’ve described two thing that specifically
aren’t balanced, not even in their own native game. The Vindicare Assassin is virtually invincible as he Dodges on a 110 or lower usually, can Dodge anything regardless of source, and can Dodge multiple times a turn (around 6-ish). The Primaris on the other hand can cast powers that have tiny Thresholds with a large amount of dice, racking up overbleed like you can’t even imagine, all with Unnatural Willpower and other things that make him a walking maelstrom of total death and destruction.
You can combine Deathwatch, Dark Heresy and Rogue Trader... but I don’t think Ascension should ever come into it. Just combining the main 3 shouldn’t present too many problems apart from the differences between low-to-mid level
DH characters and
DW Marines. Make sure you’re using the Deathwatch errata for the new Marine weapon stats, but I wouldn’t have any
DH character under Rank 5 involved with these sorts of characters. Also be careful of Hordes. Marines can deal with them. Squishy humans can’t.