As a long time mid player, I can say that the AP Trist thing messing up mids is decently accurate. It's less to do with champs than it is mindset, though. Throwing AP Trist mid is like throwing Nunu mid; mid players are used to constant stress looking for any advantage they can take, timing their own cooldowns as well as adapting to the cooldowns of the enemy, paying attention to damage at different stages of the game, etc. Usually mids will throw out abilities just to see what will happen and get a feel for the playstyle of the opponent before they go all in around 6. However, AP Trist throws a brick into all of that because she's not going to react to anything you do unless you miss an ability, in which case she'll jump to you, hit you with her DoT and then auto you down. That's extremely punishing, as it's "You better not use your abilities to farm if they don't have short cooldowns, and you better not miss me because then you're porting home after this or you're dead." That can be said for most mids when there's an exchange that's going to go down, but Trist can capitalize on almost anything. Mids aren't used to that, even if they play other lanes, because they went into the game expecting the mid mindset. Nunu is similar in that he's not very reactive, he just gets in your face, snowballs you, eats a creep, backs off and is usually tanky enough and has enough sustain that he wins every single exchange. I'd have to say that neither of them are necessarily better picks than most other mids teamcomp-wise. Your main strategy is to keep the enemy mid off balance mentally, punish them hard, keep them underleveled, don't let them roam and set up an advantage for your team to capitalize on. If the game lasts late into the game then the enemy might be able to recover from this and be in a better position even if they've been losing most of the game.
My experience from three seasons, anyway.