I played the early access 1.04 version.
If the title of the game didn't already clue you in, Sexy Mystic Survivors is a Vampire Survivors clone with sexual content. Help the sexy and mystic girls survive the horde of the enemy and be rewarded with favours of the best kind.
Like Vampire Survivors, you run around a mostly empty map defeating minions, gaining stronger weapons and bonuses until you survive for roughly 30 minutes. Use gold collected during runs to permanently upgrade your characters, and unlock new content by completing achievements and trying new characters.
Despite minor gripes such as the early game being annoyingly limited, and what difficulties you can tackle based more on your out-of-run progression over your skill in-game, SMS plays well and adds enough to the formula to stand on it's own two feet.
if you've already played what this game copies you will find a fresh experience with some neat changes, and some animated CG scenes when you complete a run.
The sexual content is unfortunately very limited. The first five girls have 2 CGs each, and absolutely nothing for the other four. They're animated and you can jiggle their titties with a click but if a total of 10 CGs after several hours of learning and grinding through the game don't interest you, then don't bother. However, you might find that this game will have something more to offer than just hentai - gameplay.
If you're having trouble look through the steam guides to get some pointers. Don't be afraid of casual mode.