is that there are no hooks into Windows core
There's nothing wrong with API hooking as long as it's done properly however it depends on what you want to do, since sometimes there are better and documented methods of doing something. The problem is when hooking occurs but is not done properly and leads to a big slow-down and crashes.
As for hooking into the Windows core, most security software that does this for purposes such as virtualization actually work with the hyper-visor, allowing them to forget about PatchGuard being present on x64 systems, because they'll still be able to make use of kernel-mode hooking (e.g. MSR hooks). Therefore, the hooks won't be valid system-wide into the real Windows core of the OS (since PatchGuard would prevent it on x64 at least), but the hyper-visor is being used system-wide for real virtualization.
That being said, there is more to hooking than kernel-mode hooking... User-mode hooking can also occur, and that doesn't touch the Windows Kernel and is applied from user-mode.
Microsoft have actually made a library for API hooking (user-mode), it's called MS Detours; the free version supports x86 processes only, the professional version supports x64 processes too (but costs a lot of money). I assume there are tons of vendors out there using it if they apply hooking for behavioral monitoring and the such.
Obviously supported and documented methods are much better, but like I said, depends on what you want to do. ReHIPS haven't taken the approach most others take but then again, it's unique in it's own way and if it works for them then great
