"The Unicorn and the Wasp" is indeed excellent
You're insane. Roberts actually seemed to think he'd written something as clever as an Agatha Christie murder mystery. He didn't seem to appreciate that the only resemblances were cosmetic and there was no possible logic to work out who the killer / giant fucking wasp was. That episode sits right next to "Love & Monsters" and "Fear Her" as the only episodes of nuWho that I will never, ever watch again. ("The Shakespeare Code" isn't far behind ... I've never actually fallen asleep during an episode of Doctor Who before.)
While I wouldn't call The Unicorn & The Wasp excellent, I think you are a little hard on it - I didn't mind it so much. I'd never rank it anywhere near as low as Love & Monsters which I agree is really, really awful.