I had to disable Windows Defender in Group Policy before the installation would work, but, on Windows 10 Insider Build 14295, things are working fairly well. The odd hiccup here and there, but nothing traumatic. Installing WB Beta solved a lot of glitches.
Just an FYI... it's not recommended to utilize Windows Defender, as it is not effective (it never has been). Even the more improved version shipped with Windows 10 is not remotely trustworthy in preventing malware/viruses.
Been using Microsoft Security Essentials/Windows Defender since it's inception and have never had a virus/malware/trojan or anything else get through. Don't believe all the BS other AV/Malware companies tell you, they would say anything to promote their own products.
That information isn't from competitors... in every independent test conducted on the efficacy of AV programs' abilities to detect malware/viruses, Windows Defender/MSS has never broken 90% (the Windows 8 version was something like 56% detection).
Is it likely Windows Defender/MSS will continue to greatly improve on Windows 10 to the point it's on par with other competitors and scoring a minimum 95% (98.5 - 99.99% preferable), absolutely... but it isn't even close to being there yet.
I appreciate your perspective and experience thus far, however you're either extremely lucky, have good browsing and installation security etiquette, or have been been infected by malware and you simply don't know (the probability of the latter goes up substantially if you've relied only on MSS for malware/virus protection, especially if used exclusively prior to Windows 10 since you don't know what you can't detect).