Ages ago you could download binaries for Buffalo routers (g300nh / g300nh2) and flash it through the web ui. If you tried to build those images yourself, though, it would fail at the final step because the signing tools / keys were only in the possession of BrainSlayer / Sebastian. Other platforms relied on binary blobs for wlan drivers, and things like that.

Out of the two of them DD-WRT is more "hacky" and duct-taped together, while OpenWrt is more "pure". This can mean that DD-WRT feels more feature complete at times.

DD-WRT specifically includes drivers/firmware blobs that OpenWRT can't/doesn't. Mostly broadcom stuff afaik.