Let me explain every single bit of it then:
Since windows vista, a windows iso consists(mainly) of a boot.wim and an install.wim: the boot.wim(specifically index 2) is the installer PE which you use to partition your drive and initialize installation; the install.wim is what the installer PE copies to your hard drive(more specifically, its contents).
Using windows 10's boot.wim to install windows 7 means you're essentially booting windows 10 first and windows 7 after. Now, since booting windows 10 is easy because all of the native support, there's no real way of knowing if the actual windows 7 will boot and/or if the usb ports will work after windows 7 boots - windows 7 does NOT support the new controllers NATIVELY, not to mention all the possible UEFI and SATA configurations baked into almost all modern UEFI BIOSes, and you're flying blind - installing an older OS through a more up-to-date PE. So, installing windows 7 through a windows 10 boot.wim doesn't do jack for you if you have no knowledge of how windows 7 works on the specific machine BEFOREHAND. This is why I don't do any more windows 7 esd images. I've done enough testing in the past 2+ years to know that you haven't.
and you think working with DISM to integreate driver into both boot & install images is easier than a copy past?
Copy-paste is the easy way. Integrating the proper drivers in the proper indexes is THE RIGHT way. If anyone is looking for an easy way to run windows 7 on modern hardware, then I'm sorry, but there's no actual easy way to do it and still be UNIVERSAL for all machines. It's why I'll be editing my tutorial on the subject when I get the time.