As far as OS RNG's go, unfortunately /dev/(u)random on a number of systems has multiple instances of insecurity (e.g. see netbsd). The RNG in Bitcoin core is hardened against weakness of the OS rng by using a hash to combine the OS rng, hardware rngs (if available), and various sources of non-cryptographic entropy (timestamps, network counters, host info, etc.) and passes the result through an computationally expensive hardening function so that even if there is a total failure of cryptographic entropy you still have a fighting chance.
I strongly believe that somebody should make this RNG available for end-users in an external library such as libbitcoinconsensus or at least extract the code into a separate project. It seems very beneficial for secure random number generation.
Even better if it's implemented in an OS-agnostic way such as C/C++, without opening and special files in places like /proc so it can be inserted into Windows/MacOS/Linux/BSD programs at the same time.
