Without thorough understanding of the processes involved down to the first principles, one can't claim anything about physical randomness source.

As Mike said, SRAMs are remarkably good in retaining their value. Moreover, most SRAMs have some sort of unintentional but often regular (over a field of SRAM cells) asymmetry (e.g. in the physical dimensions of the transistors), which makes them prone to a bias towards one of the states upon powerup.

So, to say, "SRAMs are good sources of physical randomness" is the same as to say "throwing dice is a good source of physical randomness", without taking into account that dice are too often irregular and biased, and that an incorrect method of throwing them may lead to less than random results.

JW