When it comes to the Assassin, I can best sum up my feelings for them mech this way:
I have never, in all my years playing basic battletech, going back to before the clans were introduced, said to myself "boy, I really wish I had an assassin right now". I have never even said to myself "you know, this lance needs an assassin to round it out". In fact, the Assassin has the honor of my never (until the last 2 box sets) even owning a mini of it, something that only 1 other mech dating back that far can share (the Ostol - and that's esthetics)
Now, if I was playing a RPG, the Assassin would be high up on my list of mechs I'd like. But in the board game? nope... never used one, don't see any reason to ever use one.