There is no problem with Assassin having two NPs, per se (and I actually like the profile). The mind control can be acceptable if properly monitored during PC interactions. The thing is that there are other Servants that would also deserve a second NP to be on par in that case (Like Zerker and maybe Archer) and then It'd be unfair to deny them the right as well. Not to mention you trample on the Rider Class special NP count. 

Out of curiosity, was there not another option submited for an Assassin? Is that a no-go?