In a recent game, my level 4 valkyrie's stack (consisting of siege engine, wizards and cav) came upon an enemy city garrisoned by an high level assassin, and containing a fairly large number of units (10-12). I immediately knew my valkyrie was dead. Attacking the city was death (because of the large number of ambush units making my superior strength irrelevant), and retreating to my own city was death (my valkyrie would receive no bonus in defense). I ended up retreating and being killed.
The valkyrie already has glaring weaknesses (no defensive bonus, vulnerable to archons) does she also need the added humiliation of being totally outclassed by the assassin?