While hearthstone may be fun, I don't think it's existence speaks to the value (good or bad) of the game engine it was developed in.

It's technical excellence (esp. w.r.t. rendering) was low relative to the competition the last time I checked (~ 1 year ago). If Hearthstone is great, I suspect it's because the game designers (the people who make something fun) are good at their jobs.

Really, I suppose it comes down to the strengths of your team and your goals (fun, mind-blowing visuals, dev time, etc.). I'm sure there is a place for quite a lot of game engines to exist given the trade-offs involved.