There's also the fact that assassins can't automatically climb over walls. There's a "scale walls" ability that they have to have in order to assassinate anyone inside. (It's the scalewalls tag in the inspector)
If you were going to assassinate my pretender - in most games, I would be slightly annoyed, at best (But I almost always take imprisoned pretenders). I would also probably have teleported him to another fort as soon as you assassinated the first researcher in the same fort as him. So I'm not sure how well that would work out. Especially if I started bombing the province with mind hunt the same turn I teleported him out. (Not that I always have mind hunt :V)
If assassinating enemy pretenders with your own pretender was a viable strategy, I'd wager that the Teotl of the Night would be better because (a) He's already an assassin and has stealth 70, so you don't need the black heart or the amulet of the dopppelganger, (b) He can fly, so his victims only get one round to cast before he pops over next to them, (c) He can carry a normal suite of items if you do decide to fully kit him out.
Of course, that's purely theoretical, because he doesn't have scale walls. Maybe the target will come out - if they're a combat pretender, or to site-search, or w/e. Even if you had a guaranteed plan to take them out with your assassin pretender if they came out of their castles, you're still wasting turns your pretender could have done other things, just on the chance that you might be able to waste their future turns (and they'll probably just call him back).
I wouldn't do it, personally.
If you wanted to assassinate pretenders, you'd likely do better to look for national commanders with scalewalls.