L&LD
Part of the Furniture
Remember the RT-N66U... The upgrade from 5.100 to 5.110 broke a lot of wireless clients from Intel, and the following upgrade to SDK6 (as the "fix") reduced wifi performance, most likely because the hardware wasn't really tuned for the newer SDK (a lot changed between SDK5 and SDK6).
So no, newer is not always better. The SDK has very little to do with security, it's more low-level. Security will become an actual asset once Broadcom decides to finally upgrade the kernel with a new SDK.
I didn't see a 'lot of broken' clients (Intel or otherwise), and the latest firmware is better today than the firmware when it was on SDK 5x for my customers who still have the RT-N66U (and when I had it too).
I smiled when I read your last paragraph. You contradict yourself (even if I think I know what you want to convey).
I agree newer is not always better, initially, but in the long run, it is.