You can't really value the game unless you delve deeper into it. If you're simply a casual mobile gamer, then you'll hardly get any real value compared to playing a main title. The game is a lot about investing your time into building your units, and that's really fun, given the amount of creative customizations you can go for. And that said, the difficulty and challenges start rising as the skill system gets more and more complex, so there's more strategies and the harder modes become a lot more challenging.
The fun is in the customization I mention, but also the insane variety of characters and different versions of them, as well as weapons and other things. And even as a simple mobile game starting off, it does fine as a "Mobile Emblem", until you start digging into the deeper aspects of the game.
The style itself is also really charming, and not just the art. I really liked the chibis from Day 1, and they're what breathes life into the game as it's own thing.
And for the record, the game is very F2P-friendly, as someone who's played since launch barely spending any money. Every single one of the units I +10'd (including 4 5* exclusives), I didn't have to pay anything for it. I just spent a decent amount of time saving orbs, and I got about 80-90% of what I wanted in the game.