Don't get me wrong, I still enjoyed it, but it does seem to be the case that many people don't like it as much, so I thought I should point out that it's different.
Oh yeah i agree with that (though not with the sentiment, just that people didn't like it as much).
The gameplay felt considerably easier and simpler compared to the first two instalments and SS3. The hordes were indeed probably bigger towards the end, but I found that I didn't need to use various weapons as much as each kind was more effective, so long as getting overwhelmed by the sheer number wasn't a problem. Early on the shot-gun seemed pretty effective against anything, the vehicle sections were rather easy, the minigun had more ammunition, allowing the player to hold back hordes regardless of their kind for a long time, and the arenas mostly seemed to have a simpler structure—open, even fields with a clear line of sight, with a few specific areas like the level in the trees. I found the difficulty to pose some challenge only in the Kleer valley and in the final battle. Overall, it just seemed like the game was more about holding back the onslaught of large numbers of enemies with whatever weapon rather than having to engage in a more elaborate and chaotic back-and-forth with varied groups of enemies.
Perhaps (i do not remember thinking it was any easier than the earlier games when i played it but that was ~15 years ago - the only thing i do remember is that i liked that the shotgun would now kill Kleers instantly since that was by far my biggest annoyance in the earlier games), but that doesn't make Serious Sam a casual game, it just makes it a bit easier than the other games. And TBH what you describe with "
seemed like the game was more about holding back the onslaught of large numbers of enemies with whatever weapon rather than having to engage in a more elaborate and chaotic back-and-forth with varied groups of enemies" is basically what i find all Serious Sam games about :-P. Though TBH i was never a big fan of Serious Sam both because of how repetitive the combat gets and how i just dislike the arena-based gameplay (which is also why i didn't like Doom 2016 as much as other people did - to me it felt closer to Serious Sam than Doom).
Ironically even though fans most seem to consider SS2 the weakest entry in the series, for me it is my favorite because of all the variety it has in terms of environments, how it tries to spice up things with the vehicles (i had a lot of fun with the "UFO" with the spinning blades) and... well... i found most of humor funny, at the time at least :-P.
About the open fields, i think this was largely because they were showing off their engine and large open terrains with equally large draw distances were considered high tech at the time (also see Unreal Engine 2, Halo, Fable and everyone favorite, Oblivion). It also makes sense because their original demo - which is basically what put them on the map - was the Karnak Demo which is basically a big flat map with walls in it to form arenas.
As for the consoles part, an Xbox version was developed alongside of the PC version and I doubt Croteam had the resources to make two very different versions that would be adjusted for the limitations and possibilities of the platforms. The levels seemed to be made up of smaller sections and I think there were more loading screens, too. What I mentioned about the hordes being more homogeneous and attacking in more even clusters would also be something more suitable for handling with console controls. I could be wrong about this, but flying enemies or enemies placed at different elevations were a rarer sight and were a bit more compartmentalized into their own sections.
No there were a lot of flying enemies, a variety of heights and IIRC even some platforming. I remember the levels to be large enough but this was never much of a thing with Serious Sam games since after you clear an arena there is little reason to go back. There are also areas where *you* fly up and down.
Besides, i think you overrestimate the effect having to support a controller can have on an FPS game - outside of some form of aim assist/autoaim and a UI that is readable from distance and isn't too complicated to use (which isn't that hard - even Morrowind's UI worked on Xbox with a few modifications) you do not need much. And even aim assist isn't really a must, just a nice to have. E.g. i can play Quake 1 via Quakespasm (which supports controllers) just fine with a controller, it is less accurate than a mouse but you can still play just fine after you get used to it. And that is a game with a lot of vertical areas, flying/jumping enemies, etc.