"The Authorized Dinan Dealer will, without charge for parts or labor, repair or replace the defective Dinan component(s), as well as any original vehicle manufacturer's component(s) that may have been directly affected by a Dinan component, using new or authorized remanufactured parts"
So if BMW blames Dinan for some failure caused by their part, and Dinan disagrees, you're stuck in the middle having to pay for the repairs, but how common is that really? There are lots of rumors online, but the only clear case I could find is for someone who had a problem after racing their car, and the Dinan warranty specifically says that they will not cover cars which are "operated in any competitive event". I'd be interested to see other examples.
This still blows away the other 3rd parties out there which are not taking any responsibility for the damage that could be caused by their parts, in fact some of them go as far as suggesting that you remove their parts before warranty claims to avoid being denied coverage, for example from a popular tuner:
"we suggest removing any modifications prior to making a warranty claim. This is to ensure they are not causing the problem". If their part is causing the problem, removing the part is not going to magically cause the part to not cause the problem. This seems more like encouraging the customers to commit warranty fraud.
Personally, I won't give my money to companies with such unethical behavior. Dinan still looks to me like the best option out there, as long as you are ready to pay a premium.
Edit: I just checked out AC Schnitzer and they seem to provide a similar warranty to Dinan, kudos to them as well. It might be a bit trickier just in case something goes wrong since they are based in Europe. I also don't think they go through CARB certification which is required for the folks living in California. So my preference would still be for Dinan being in the US.