Eternal's gameplay is simple in my opinion, I have played many, many hours of it. It is a bit strategical of what firepower you use in a situation but it's pretty simple in overall, and I like it! Now going through the second DLC
I don't think eternals combat is simple at all.
It just does such a good job of teaching it's mechanics that it ends up becoming second nature.
You have 9 weapons, 2 alt fires for each, 15 to 20 different enemies some with weak spots, some weapons are appropriate for enemies but not for others.
Then you have chainsaw, blood punch, freeze bomb, grenade and flame belch to manage your resources.
The movement is very complex with double jump, monkey bar, air dash and meat hook
Like that's a lot of mechanics for a first person shooter! Most players likely already played doom 2016 so some of these mechanics were likely second nature.
If you introduced all those mechanics at the start of a game most players would be overwhelmed.
Eternal does a really good job of introducing each mechanic slowly while subtlety teaching the player the basics.( And sometimes not so so subtlety!)