There is less information in the 1939 Register than in the 1911 census; no birthplaces, or relationships within a household, naturalisation or nationality status, or fertility. Only name, date of birth, marital status and occupation were recorded, at the address where each person was on 29 September 1939.
Records are not opened or retained according to a single set of rules, and there are plenty of inconsistencies and anomalies. Legally, any census from 1921 onwards is covered by the 1920 Census Act. Prior to that, each census had its own Act, as did the 1939 Register. The 1841 and 1851 censuses were released in the 1920s, the 1891 census for Scotland was released some years before the 1891 census for England and Wales, and for a few years you could see the 1911 census for Northern Ireland in Dublin, but not in Belfast. The 1911 census for England and Wales was released early, with restrictions, as the 1939 Register for England and Wales will be, following successful challenges under FoI but the 1911 census for Scotland remained closed for the full 100 years, although it was actually opened exactly 100 years after it was taken, in April, and not in January of the following year, as is customary. The 1939 Registers for Scotland and Northern Ireland remain closed, but FoI requests can be submitted for individual records. Personal records of people who served in the armed forces after about 1923 are still held by the Ministry of Defence, but Merchant Navy records (where they survive) are open. That is what I can remember without looking anything up - goodness knows what else I'd find if I really looked hard!
And I haven't even mentioned BMD records...