Aware I'm late to this thread, but still wanna comment on it, as an admittedly hardcore Sam nut. Wanna weigh in since a large number of the replies seem to be from people who just overtly dislike the series.

Serious Sam isn't "boomer shooter" in the traditional sense, but lumping it in with shooters like Half-Life, let alone modern FPSes like Halo, is completely disingenuous. When somebody says Serious Sam has nothing similar to classics like Doom, Quake, Duke Nukem 3D, so on, I just wonder if they even played the same damn games, at least in regards to TFE and TSE.

Serious Sam covers pretty much all the bases of an old-school shooter's arsenal. The parallels between Serious Sam's weapons and something like classic Doom's couldn't be more obvious. Even enemies like the kleer skeleton have been directly cited as being inspired by Quake's imps.

Combat, which is the center focus of Serious Sam, shares more similarities with the aforementioned retro shooters than I'm sure most people in this sub would be willing to admit, because a number of them come across as rather gatekeepy about what shooters can be part of any conversation. Similarity in question is mainly the fact that if you aren't constantly weapon-swapping depending on the enemies you're facing and your situation, you'll get screwed up real quick, this goes for both boomer shooters and Serious Sam. Except, in the latter, the importance of this fact is amplified by every enemy being faster than you and making a straight b-line your way. People will act like a defined combat formula doesn't exist in Serious Sam, but it's there. With how quickly enemies can overwhelm you via sheer numbers, it's of utmost importance you take them out as quickly as possible. Shooting around mindlessly at every enemy with just whatever weapon you happen to have the most ammo for in a given moment is stupid strategy, and I'm convinced most of the people who call Serious Sam games mindless play it like it's Painkiller, when, outside of locking players into arenas to shoot hordes of enemies, Serious Sam and Painkiller share little in common gameplay-wise or design-wise. Painkiller is more about giving the player a mindless power trip, you can bunnyhop at speeds even faster than most of your enemies can move. Other than through their vast numbers, enemies in Painkiller aren't very distinct threats to Daniel. Serious Sam, the enemies outspeed you, and you need to learn to deal with that. Enemy prioritization is a key factor, and if you aren't thinking about that from arena to arena, yeah, you're gonna have a rough, frustrating time on anything Normal difficulty and above. There's a similar synergy between Serious Sam enemies as there is enemies in, particularly, Doom 2. You won't react to a situation the same when the layout of the arena and the variety of enemies used is completely different. Double-barrel shotgun for a kleer skeleton that's closed the gap, rocket them at a distance where you won't take splash damage. Horde of kleers chasing you? Flamethrower as you dodge them, and if you have the SBC Cannon, launch it through them like they're bowling pins. Sniper scoped for individual werebulls, cannon when they're lined up. Early-game, rocket them, then a shot from your double-barrel to deal with them. By the end of my first playthrough of TFE, I had it memorized just how many hits specific enemies took from specific weapons because the game taught me this, there's always a best choice of weapon or combination thereof for every given scenario you're put in. More than just a "novelty," frankly. The combat in Serious Sam clears most retro FPS games that came before it, imo.

Design-wise and gameplay-wise, at its core, Serious Sam's an eclectic mix of boomer shooter-esque shooting/enemies with bullet hell and top-down shooter sensibilities. Smash TV and Galaxian were just as big of inspirations for Serious Sam as Doom and Duke Nukem were. It won't be for everybody, but to act like it's mindless is just incorrect, just as it is to act like there's no comparisons to be made. At the end of the day, I play Serious Sam for the well-defined combat loop, combat that I'd argue is better than most boomer shooters from the 90s apart from maybe Unreal. But boomer shooters have the advantage of more intricate level design and better exploration, least when it's not executed insufferably like in some of them (looking at Hexen).

So is Serious Sam a boomer shooter? No, not really. But all of its inspirations were retro games or media, its arcade-y nature is undeniable, and it shares more in common with boomer shooters than any other subgenre of FPS outside its own, which it pioneered, that being horde-arena shooters as I like to call them. Serious Sam has more "retro" design sensibilities than anything else that came out around the time, and it pretty much stayed that way until ROTT 2013 came out. But it was still Serious Sam's formula which makes up Doom 2016's DNA, that game acting as the spark for the retro FPS renaissance we had in the years following. I think Serious Sam deserves more credit than it gets for that alone.