Personally I've used WD Red 4TB and 6TB drives and haven't had any issues. Admittedly, I installed Debian on mine and haven't tried it out with the stock firmware but imagine it would work just fine.
In my case I usually don't have all 8 drives in at the same time so I haven't really tested whether the device can handle that from a heat/power perspective. You should be able to find info on the power requirements of the drives you have and compare it to the ones you want to use to get an idea whether you need to worry about that.
It's hard to get a definitive answer for whether an older device will support newer drives, there are a number of things that go into it (and it changes over time). For this device:
The chipset can definitely handle >2TB drives (some older chipsets couldn't)
The chipset can handle 4K sectors (some older chipsets couldn't)
The firmware can handle GPT partitons, so the 2TB MBR limit isn't an issue
It has a 64-bit CPU so the OS isn't limited to 32-bit addresses which limit some file-systems to 16TB
That said, I do not know if the stock firmware for this device can handle filesystems greater 16TB. It might not.
I've never looked into drives larger than 8TB and don't know if there are any issues related to them specifically. Each of the issues I mentioned above I learned about the hard way.
The best thing I can recommend is to test out your configuration thoroughly before you depend on it. Usually I:
Add the drives and make sure they report the expected capacity
write zeros to each drive and make sure the OS can write the entire drive and that the performance is reasonable
Create whatever raid arrays I want and make sure they report healthy and the proper size.
write zeros to the array(s) and make sure the OS can write the entire array and that the performance is reasonable
create the filesystem on the array(s) and make sure no errors are reported.
fill the filesystem with random files and make sure the OS can write to the entire filesystem and that the performance is reasonable
