I think with a game like BL2 you really need both.
A lot of the typical FPS mechanics go into it, but skills and unique weapons really throw a wrench into it.
An example of this is the differing mechanics on damage multipliers and where numbers get plugged into the damage math. A player just looking at crit modifiers or a skill tree (melee Zer0 comes to mind as a real example of this) isn’t going to have any way of figuring out why gun/build A is performing so much better/worse than B.
I’m very thankful for the people who provide statistical analysis around these parts because I’m not if the mindset to spend hours plotting data points to come up with all the info I’ve come across here.
That being said, I’m also not one to go and replicate exactly a build posted here, but reading about how and why synergies work frees up more time I can spend tweaking things I know will work while having fun.