I have been playing for over 4 months now, and I have missed one single day of doing every daily quest. In all of that time I have summoned three of the many legendary units. The grind is absolutely insane if you want to be relevant in any of the more difficult levels of the game. The Devs probably hear this all the time, but of all the games I've played with the ability to summon legendary units, this is by far the most punishing. This is as close to pay to win as a 'free to play' game gets.