It doesn't hurt that it's one of the more generous gatchas out there. Like, I don't think I've ever had it try to push transactions on me and it's just constantly handing out the currency needed for pulls.
Plus the monthly bonus thing is, like, $5. It gets you 90 primogen at daily login (requires 160 for a pull) for 30 days. It's also an at-will purchase not a subscription, no pressure to keep spending.
That said, the level up grind is very real. It takes numerous resources to level up characters, their talents, weapons, artifacts, etc. You will find that, the more characters you have, the much harder it is to maintain all of them.
Now they make it relatively easy to get material to do all this (mostly through short dungeon battles, enemy drops, or boss fights), but not everything is available every day and you'll need to be sure you know what you need for who and where and when to get it so you don't wind up flailing around pointlessly.