The spindle is too small to adapt it to R8 collets, in my opinion anyway. It probably uses #2 Morse taper collets, which are readily available, so tooling it isn't necessarily a big pain. A set of off-shore made collets that range from 1/16" up to 1/2" can be purchased for less than $50 US. Is the only reason you might want to use R8 collets is the extra capacity for bigger tools? Because if you need to spin tools with 3/4" or 1 " shanks on them, then this isn't the machine for you anyway.
As far as breakage goes, I think the most vulnerable spot on these seems to be the quill fine-feed mechanism, there are 3 gears that are used to transmit power from the manual handwheel to the quill, and there's one of those gears that will strip if a person overloads the fine feed by trying to force-feed the tool instead of using the rapid feed handle for heavy jobs. Replacing the gear can be expensive, but there is a cheaper alternative for the repair that involves buying a commonly available gear from McMaster-Carr and doing some alterations to make it fit.
At this point I'm not aware of any other real 'weak points" on this mill. Its like any other mill, in the sense that wherever there are parts that move, rotate, or transmit power, there's something that needs to be maintained and/or lubricated. There's plenty of good reference material to leaf through online if you want to find out more. Here's one of the better spots to read up on the 8520 milling machine...
http://www.wentztech.com/metalworking/equipment/clausing-8520-mill/
Whether this mill is a better option than a Grizzly or any other machine - I think it boils down to a person being able to make intelligent decisions how to go about the job(s) they need to get done versus the limitations of the machine, then decide if you need or want to confine yourself to a certain budget figure. With respect to the asking price of the one you're investigating, somebody with more experience in that area may be able to offer some guidance.
All I can add is that now that I've got my Clausing set up and running, I'm happy as Hell that I don't have to do without it.
Hope this helps.
Best,
Chris S