It's just not that fun.
Serious Sam's weapons aren't fit for a rougelite experience, they're by design basic weapon archetypes, and this game doesn't even try to mix them up. What makes Serious Sam games fun is learning the enemies and then utilizing the weapons in an optimal ways depending on the enemy types and arenas.
Here - there's just no room for that. Firstly, unlocking the weapons is a part of the meta progression. You start with only Pistol-Shotgun-TommyGun available to you and, judging by how much currency I got, are expected to play for at least 2-3 hours (assuming you complete the runs) before you can unlock a single new weapon. And even then, being a roguelite, the game will dispense the weapons at random, so you're still gonna have to use whatever the game has given you against all kinds of enemies and groups you'd normally dispatch using something else. And you can't even expect to have all of them by the end of a run, as you often just get repeats with slightly altered stats.
And that's the main issue with the game, really, the whole concept just doesn't work even on paper imo, and it doesn't really try to solve these issues with anything else. Upgrades you find are super basic, the typical, barely-noticeable '3% more whatever' stuff. The ultimate ability also gets only the most basic incremental upgrades. I can already tell that, weapon order aside, every run's just gonna feel exactly the same, and that's a huge problem for a rogue-lite. The levels also lack any interesting layouts, mechanics, or even visuals, they're just flat, open spaces, with nothing unique to differentiate most encounters.
Graphically, it just looks plain. I don't mind the artstyle itself, but it feels like there was no thought put into visual clarity. The environments are both plain and boring yet overdetailed, and the flat lighting makes enemies often just blend in with the environment, especially the kleers, which seem to have to shading to them at all.
The UI is terribly cluttered, and when you switch a weapon, no matter how many you have, a huge 8x3 grid shows up right under your crosshair, as you end up with like 3/4 of your screen being covered with useless info. Like, I don't need to constantly see how much money I have or time I spent - and there's no UI customization that I could find either.
And then there's the other stuff, like characters being annoying and talking constantly, shaders loading before every level (and there's no justification for this game to have loading times like these) etc., but honestly if the gameplay was fun I could easily forgive those hiccups. But it just isn't.
Edit: Gotta vent about my pet peeve - characterization of Retro Sam. They turned him into a joke, where the one liners he says are supposedly his alien language, and all have deeper meanings that have nothing to do with actual words being said. That's dumb already, but then as you play, he keeps saying those same oneliners as just oneliners. And then sometimes he talks normal with the team too. So which is it? I get that it's supposed to be a joke, but good jokes are actually clever and consistent. This is just 'lol random'.
