If you reduce it to its bare forumla it's exactly what you're thinking, running missions over and over again to level the 4 classes, gain new weapons/gadgets/perks and cosmetics for each class and then running those same missions on higher difficulties to get better rewards. Think of it kinda like Diablo gear farming except you have a clear progression path laid put before rather than RNG.
So if the above is not your type of experience then yeah, probably not worth your time.
That being said, I've been addicted to the game since it hit PS Plus, it's wonderful and full of really smart design and fantastic class synergy/coop. ROCK AND STONE!