How revolutionary Serious Sam TFE was, as compared to its FPS contemporaries? (late 90s -2000s)

I watched some interview videos on Youtube before, and the developers of Croteam said that Serious Sam TFE was a unique game, because it has vast open spaces, bright colors, along with having large numbers of enemies on the screen all at once.

These features were no doubt groundbreaking at the time, judging by late 90s - early 2000s standards.

Davor Hunski also said that he was influenced by the 2D old-school FPS games, such as Doom or Duke Nukem.

However, I don't have any experiences with Boomer Shooters, so it is difficult for me to conceptualize or understand how big of a deal Serious Sam was at the time.

Of course, I was also very young in the early 2000s, so I certainly didn't know how video games work or look like. I played TFE a bit as a kid, but I didn't progress further into the game. The steps at the temple of Hatshepsut was the most memorable scenery for the younger me, combined with the dark indoor hallways, but that's about it.

In what way do you think that Serious Sam TFE was revolutionary or groundbreaking for its time?

For those who played the game for the first time in the 2000s, what was your reaction? Can you list out the features, designs and mechanics that made Serious Sam unique for its time?

Thanks in advance!