Is any decent RNG acceptable as a stream cipher?
The property desired of stream ciphers, just like for any random number generators, is indistinguishability from true randomness &mdash; and indeed, any RNG that fails a statistical test suite is obviously not a good stream cipher.

However, to be considered secure, a stream cipher must not only withstand a generic battery of statistical tests; its output needs to be indistinguishable from a truly random bitstream even for an attacker who knows exactly how the cipher works (but doesn't know the key), allowing them to devise specific tests just for that algorithm.

This is the point where most non-crypto RNGs fail: there are various cryptanalytic techniques than can be used to devise specific tests to tell the output of a given algorithm from random data, and RNGs which are not specifically designed to withstand them will usually succumb to them.

Of course, as the state of the art advances, sometimes that can happen even for ciphers that were previously thought to be secure, but any cipher generally accepted as secure must at least have withstood a considerable amount of attention from the cryptanalytic community without being broken.