Great question. I think for hard use, I hear less issues with milled slides. I have been running a P09 with a milled slide and SwampFox optic for a couple years now with zero issues. Also, my P365XL has a Holosun direct mounted and no issues. My only other optic pistol is a Beretta Elite LTT with Holosun and so far no issues. Like someone else said, there are so many optics ready models these days. I think the plus with optics ready is if you have any warranty issues, you should be able to send it back for repair with no problems. Once you have milled the slide, I think most companies will not help you out.