Mercenaries is fine if you want to play PvE and gradually collect / level up your team, for your own satisfaction. I have actually been enjoying doing that F2P.
But if you want to compete in PvP, it is pay-to-win trash, in my honest opinion. It feels much worse that HS in that regard.
The coin system is particularly toxic, because you have almost no agency over what you can craft. Unlike HS, where you can save up dust to craft your choice of Legendary, in Mercs you are totally at the mercy of coin drops for a specific hero. It feels terrible.