The simplicity of the ending is to it's strength in my opinion, it avoids Hollywood gung-ho guns blazing culminations and opts for a more soft ending which fits to the story and Kates journey.
You've also had a bit of mystery around the Sicario himself diminished by watching the sequel first
(albeit accidentally) I feel. If you have a bit of time to kill...watch/read a few articles online about how Kate is actually the villain of the piece.
The cinematography is stunning...the scene where they descend into the mine is like something straight out of a chilling horror movie. The use of sound, especially when they cross over the border and return on the bridge unnerves me and makes me feel sick with anticipation, despite watching multiple times, again to the movies strength.
It does feature the holy trinity of director, cinematographer and composer...Denis Villeneuve, Roger Deakins and Jóhann Jóhannsson...shame we'll never see a collaboration like that again.
I really do like Sicario, I think it's a modern masterpiece, to me it's near perfect and it sits in my top 10 movies of all time, the sequel would just about scrape into a top 100 perhaps. The sequel while a decent stand alone movie doesn't add or improve anything on the first movie.
Have you watched the other movies in Taylor Sheridans Frontier trilogy...Hell Or High Water and Wind River?