I spent whole day trying to solve the problem with either windows 8.1 and windows 10. I managed to install windows 8.1 once, but I run into some issues after upgrading to windows 10 and decided for clean reinstall. Since then, it never worked. I even ended up formatting swap EFI accidentally so I wiped whole fusion drive and reinstalled el captain from the internet ...twice. Once myself, second time with support on another computer's chat.
Few highlights from windows 64-bit installer (both 8.1 or 10):
0) Boot Camp Assistant 6.0.0 (519), El Capitan 10.11.1, iMac late 2015, 2GB fusion
1) Windows can't install on partition prepared by Boot Camp ("Windows can't be installed on drive 0 partition 5. (Show details)" "Windows cannot be installed to this hard disk space. Windows must be installed to a partition formatted as NTFS.")
2) When I selected BOOTCAMP partition and did Format (alt+f), Next button became enabled, but when pressed I got different error ("We couldn't create a new partition or locate an existing one. For more information, see the Setup log files.")
3) Deleting partition and creating new one automatically creates 4 partitions, (error from 2) remains)
4) If you decide return to mac and undo action with boot camp assistant, first remove all automatically generated partitions by windows installer (in 3) or you won't be able to revert without fdisk (some bootcamp assistant error)
5) When you do run boot camp assistant again (after 3) and 4), it displays primary partition smaller and offers you another install (like you lost some space), just proceed and when you'll be able, cancel (stop button). It'll revert your partition to original state and full size.
6) If you choose another approach and just delete BOOTCAMP partition from MS installer and press next without creating New partitions, you'll get one step closer. 4 partitions create without telling you, you'll get to next screen with "Copying Windows files (0%)" but after 10s another error: "Windows detected that the EFI system partition was formatted as NTFS. Format the EFI system partition as FAT32, and restart the installation."
7) Regarding 6): DON'T ever touch any mac or swap partitions via windows installer, NTFS error is ********, you'll end up repartitioning reformatting whole disc if you listen to MS
Now I should find a way how to create MS10 bootable flash drive I suggest...