Brute Squad -- For some reason, +1 Card on a terminal just feels wrong to me, like it's more of a penalty than a bonus. It's such little draw, but it still means you can draw an action dead. Just feels weird. That's a personal thing though.
Attacking other attacks is really weird. If Brute Squad is the only attack around, there's very little point in going for it. Might as well just get Silver. But if there is a strong attack on the board like Goons or Margrave, Brute Squad gets powerful and also extremely swingy. Unfortunately, what may happen is that people just decide to skip attack cards entirely because of the threat of Brute Squad, and then nobody gets Brute Squad either because nobody bought any attacks.
Hired Assassin -- Seems way too swingy to me. It's swingy on more than one level -- you have to draw it with a good target card, and your opponents have to have that card in their discard. Like, your opponent draws his Assassin with a King's Court and trashes your KC. You draw your assassin with nothing but Coppers and Estates, or you draw it right after your opponents have all reshuffled. It's just such a big gap.
A suggestion to fix that is that you have to discard the card you are targetting. So if you want to trash everybody's Gold, you have to discard a Gold from your hand. That also means you can't play KC-HA and reveal the same strong card three times.
Swinginess is not necessarily a deal breaker, but a bigger issue for me is that this becomes political. Say you draw Hired Assassin with two different good cards. Gold and Margrave, maybe. You've kept track of your opponents' discards and you know that one has discarded Gold but not Margrave and the other has discarded Margrave but not Gold. Now you can choose who to attack.
If you are OK with potential politics and high swinginess, then it still works. It's not my cup of tea though. Still, the most important thing is that you and you gaming group enjoy it.