Just as joe bloggs said : the "unlock 2 event chests" can cause an event to stop ( not permanently but long enought for it to end). To achieve 2 chests with only 25 coins profit takes about 100 fights. Well , if you got to a point where tower lvl is to hight for you, the amount of fights gets doubled, eventually stoping you from geting further with the event (the only solution is .shop, but that would be a litlle too much for most players)
The event quests are nearly filling one day to fullfill one quest each, if you want to unlock legendary room.
I had 5 Quests left, then: Find 250 Epic Cards. Next quest: find 250 Event rare cards: (Faith Healer),
If you do not spend gems to reload, you have to play 13 times against the 2nd event openent.
Means 13 h waiting, nearly imposible if you have a job and real life.