I’ve experienced the same issue...I have the latest 2767-20 Milwaukee with claimed 1400 lb-ft nut busting tq. I ran into some over torqued lug nuts a couple months ago on a truck, and to my surprise, the Milwaukee would NOT take them off, even with a shortie 1/2 impact socket! We tried another guy's Milwaukee, same result.
Another tech with the latest Snap On 1/2" impact (CT-9075 I believe) rated at 1100 or 1200 lb-ft took ALL the lug nuts off without hardly an extra second of hammering! I tried for 8-10 seconds straight, multiple time with my Milwaukee, and they didn't budge! Needless to say, for once.....I was VERY impressed with Snap On! Now.....I couldn't justify the $850 or whatever that thing cost for the extra punch (compared to $299 for my Milwaukee!), but I can say their cordless impact is the real deal!!