Well, here is my story with this M5210.
I ordered one from
amazon about the time this tread kicked off for $80 figuring I'd probably have to return it. It showed up with a battery AND a 2GB cache module. But, it had no back plate.
I emailed the seller asking about a back plate. He responding saying he was sorry, but if I wanted to keep the card he'd refund %50 of the price. I tested the card to make sure it wasn't DOA, and at that price I figured what the heck so I agreed.
I ordered a backplate off ebay for $8.50 or so, and BAM, I have a 12G SAS controller with a 2GB cache module AND battery backup for less than $50 all in.
Under windows 10, MSM was able to flash the card to the latest AVAGO without any issue. I've been able to create 0, 1, and parity raid levels with no issue. I don't have any 12G SAS at the moment, but with 6G SATA performance seems to be limited by the pyhsical drives - RAID 0 scales well as one would expect.
They controller is just kinda 'there' in my primary workstation (which is all PCIE NVMe) and I'm not really using it (or the array I created on it) so if there are any 'test' scenarios someone is interested in ask, and I'll see if I can assist.
It does seem to be 'luck of the draw' with these cards as far as how they are/aren't 'optioned', so that is an obvious downside. My amazon listing said it included both back plates, but said nothing about the 2gb cache or battery.
-Edit-
Current platform it's in: ASUS Prime X299-A