Thermal noise random
A true random number generator (TRNG) usually consists of two components: an “unpredictable” source with high entropy, and a randomness extractor — a function which, when applied to the source, produces a result that is statistically close to the uniform distribution. When the output of a TRNG is used for cryptographic needs, it is prudent to assume that an adversary may have some (limited?) influence on the distribution of the high-entropy source.

Thus it takes a bit of work to obtain randomness and there are many pitfalls.

If it's done well, the statistical randomness of the extractor’s output can be proven, and is not based on any unproven assumptions, such as the security of cryptographic hash functions. On the other hand output rates may be slow, and ensuring outside physical influences are thwarted may not be always possible. Obviously thermal noise in a circuit may be influenced by other factors.