I'd read that Darrow and Pearce definitely thought she was miscast. She definitely overdid it both ways. Just checked the blog I did last time I did a run through of episodes and I wrote that she's too annoying as Piri and too arch as Cancer. I did argue that her performance is so camp it almost works. Almost is doing a lot of heavy lifting here. She does get the most hilarious death scene in all of Blakes 7 mind you.
I didn't mind her too much as Piri -- she was whiny and useless, but the character was supposed to be whiny and useless -- but she was unwatchably dreadful as Cancer. Didn't they audition her for both parts beforehand? Why did they even cast her?
Also, if her poisonous brooch-creature thing is meant to be a crab (Cancer), why does it look like a toy spider? For that matter, why is a super-secret anonymous assassin so invested in branding? Although maybe it's specifically so that she can use the Cancer facade as a decoy, as she did here. But then, presumably she would've had a decoy Cancer already instead of having to buy one from the slavers. (And does her black ship have weird black controls which are labeled in black on a black background with a small black light that lights up black to let you know you’ve done it? What is this, some kind of intergalactic hyper-hearse?)
Still, I think "Assassin" mostly works aside from her. It's the first episode that lets Soolin shine as a character in her own right rather than just a stand-in reading Cally's lines, and it's got Richard Hurndall as a character that you can easily imagine is the First Doctor undercover (presuming he used respiratory cutoff to feign death and for some reason had the TARDIS stowed in the hold so he could escape before the ship blew up). I also like it that Avon proves capable of altruism after all. I prefer that nuanced portrayal to the ruthless caricature of "Stardrive."
It has a lot of Star Wars style screen wipe transitions as well.
I always find it ironic when people use "Star Wars style" to describe something that Star Wars did as an homage to the style of something decades older, in this case 1930s movie serials.