As a priest fan and player, I can say that Questline Control Priest is too slow for this meta and in case you win you'll most probably do it before having completed the quest. So, why having to play cards in particular order with no turn value, instead of playing your deck based on what is needed to counter each opponent? In case you love Priest and you are an experienced (Control) Priest player, you should give this deck a try, as it seems a quite viable Priest version in the current meta.
I have a winrate of 69% with this deck and it took me from diamond 5 to legend, where I have been stuck for a long time trying different (non-meta) decks and classes. Worst matchups seem to be Questline Warlocks and Mages, in case they go too fast with their quests and you are unable to steal their quest minion with Illucia. In general, it has chances and counters against almost every deck of the current meta, but you need to be very careful, not so greedy with your spells and minions value and weigh well your decisions depending on your opponent's deck and needs.
+ 10 votes : Mulligan guide
+ 20 votes : Complete gameplay guide