Does anyone think there would be a problem using Deathwatch (or RT) creatures in a game of DH? Mostly the stats are the same (comparing DW Genestealers to their Creatures Anathaema counterparts for instance), but with a few tweaks.

Characters are level 8, preparing to ascend.

We already do for our Ascension group, as it outgrew the standart Chaos and Recidivist rabble. At the high ranks they can (and should) get their hands on bigger opponents to chew out. And their stats are for example in DW.

I got myself the DW "Mark of the Xenos" for this exact purpose! I once watched a thrice-bound DH-Daemonhost getting blasted to bits by a group of four acolytes (two of them wielding Bolt-Weapons while one of them being a psyker).

'while a lot of creatures/opponents are to powerful for a pre-Ascenions group, it might work better with Ascension. Even without "Rated: A", the book has some beasties who look like you could throw them at a Rank 5+ group with matching gear.

Do not take their Wounds 1-1 over to DH, so! Their "Krootox" comes with about 80 of it. In my books, this is simply a shame since the massive ogres (DotdG) come with about 30. So, reducing entries featuring more then much more then 50 wounds by 1/3 is something I would advise. Oh! And have an eye on the damage they put out. The "single opponent" (as compared to "horde opponent") types are damage-tailored for a marine (TB: 8) in a Servo Armour. Keep this in mind...

Getting enemies from the Deatwatch line would surely keep things interesting. But I also agree that they should probably be faced by an experienced group of brave Acolytes with good arming. In particular since I have this fantasy of running a campaign centered around the Tyranids invading the Calixis Sector (and surrounding galactic area) someday...

Thanks all, some excellent advice.

All good advice here! Also try to challenge your players combat prowess by putting them in unusual situations. Here are a few that I've apllied with various degrees of success: Zero-G, vacuum (with voidsuits that starts to leak air as soon as any damage is taken), in a ballroom where heavy weaponry would be socially devastating, having fragile people to escort or rescue, needing to take their prey alive, taking part in a mounted charge (preferrably on beasts that they had no chance to practice riding on), an enemy force that gets increasing reinforcements so great hurry is needed in completing the task laid out, across a 2km wide cover-less plaza with snipers and lascannons around it (and minefields!), the enemy is disguised as civilians at a heavily populated area, etc.

Any of these situations increases greatly the challenge that a well adapted but otherwise not that dangerous enemy poses.