I do remember the lenovo driver that installed itself from the bios - even on a clean windows install - and would snoop for months and then silently uninstall itself.
EDIT: here it is: https://en.wikipedia.org/wiki/Lenovo#Lenovo_Service_Engine
However I have not heard much about dell.
It's on battery #3 from the battery swelling issue, a lot of backlight bleed on the 4K Display, coil whine in the power supply (went through three of those), three DC jacks (might need to be a 4th soon), the infamous VRM/throttling issue, and to top it all off I had the main board replaced within the first two years because of a firmware problem preventing Windows from activating. Less of problem with the system itself, but during that mobo job Dell replaced the 512GB Samsung SSD with a Toshiba 1TB and R/W speeds dropped to ~1/3 of what they were before. Also anecdotal but a buddy of mine with a 9560 has gone through more or less the same issues, sans the firmware problem.
They look so appealing on paper (which is why I chose it over a MBP), but there's just something about them that feels proof-of-concept, like it's an unfinished product that was pushed out too quick. I wanted to give Dell another chance, having gone private three years before and having seemingly improved their build quality. When you spend $3800 on what is supposed to be the best-of-the-best and it turns out to be the headache of a lifetime, it leaves a really sour aftertaste.
That pretty much summed up the difference between Apple and everyone else for me.
I've seen a lot of bad reports, so maybe I just got lucky, but I hope not since I loved that machine and the XPS line would be pretty high up on my list if I was in the market for a new laptop now.
[0]: https://arstechnica.com/information-technology/2015/08/lenov...
[1]: https://news.ycombinator.com/item?id=19800807
[2]: https://www.absolute.com/partners/device-compatibility/
[3]: https://en.wikipedia.org/wiki/Absolute_Home_%26_Office#Absol...