@didor Honestly I tried Mentor in all kind of decks from Landstill to Delver and in the Delver deck it didn't feel right. I often didn't have mana for it and I also hardly trigger Pyromancer triggers. Mentor is a card you usually want to win the game, but you will also have troubles casting it.
I run Mox Pearl though so that makes it a bit easier for me but in my original build running 15 lands Mentor was problematic. If you are running 4 Wastelands it may not be such a problem and might be a nice addition but from the 3 leagues I played in and faced many of the UR Delver decks...those decks are actually more aggressive than mine and really beatdown and attack resources. Those decks don't have time for casting Mentor often. In my version (see screenshot above) the card is actually more playable even though it may not seem like it at first sight.