What version of SCCM are you using? SCCM CB 1802, only the latest kb isn't applied yet
Which WADK do you have installed? 10.1.17314.1
Does it boot no problem into a VM using pxe? No, no PXE in place yet
How about a system that has Win10 installed? Works fine with exact same TS and boot image
Is it only failing on systems that are older and have Win7? Yes
If yes, then there may be a driver problem in the boot image (winpe) for that system/build. Possible, yes. But the strange thing is that it works fine with a 'vanilla' build of W7. Our current suspect is our AV protection (McAfee). Turning it off doesn't make the difference, so currently trying to build a system with the regular build process minus AV.