Yes, it was the OEMs issue, but some of the media pundits and ABMers (Anything But Microsoft) made fun of NoDo update fiasco and blamed Microsoft without any knowledge of how an update would work with Windows Phone 7. Even they tried to compare it with iPhones where Apple, who is both platform provider and OEM, has control over everything. Here is the interesting finding that proves otherwise.
ChevronWP7 team found interestingly that there are at least 20 types of Windows Phones used. Most of the are labeled as the same device, but internally they are different. For example there are at least 4 types of Samsung Focus. Their Chevron WP7 unlock service collects telemetry information of every device while unlocking and that information gave clues about various configurations of each Windows Phone 7 device type. Their service logged information of nearly 5000 devices in its database. Interestingly Microsoft knows the issue and but never really blamed the OEMs for this. And Microsoft actually tried to explain why the pre-NoDo and NoDo updates were delayed and failed in some instances, they were immediately fired by the some of the media pundits. Rafael Rivera, one of the ChevronWP7 Labs founder, blogged about these findings in his blog withinwindows.com. You can read more about it on his blog here. And you could get the complete list of WP7 devices in the usage from Wikipedia page here.