First, D-Link has a standpoint that internal compents are proprietary information.
We are not required to release this information to anyone for personal use.
As for the 825 compared to the 855, what you see is what you get.
Feature wise their identical. The 855 is a 3X3 solution. That means it has 3 send and 3 receive antennas. The 825 is a 2X3 solution, that 2 Send and 3 receive. (there is an internal one.)
This can effect distance, but usually doesn't.
As for the chipset, let me say this. The DGL-4500, the DIR-855, the DIR-825 all share the same platform, we simply add or remove features/componets.
Bugs? NONE OF OUR PRODUCTS HAVE BUGS. LOL J/K. So far so good. I've not received any reports of anything too troubling. Check the forum for that product and ask around.
I hope this helps.