FWIW, this has been around for a couple of years and has felt pretty solid, i.e. no obvious rate limit when bulk data gathering. For example, I believe that if you want to collect stats at the census tract/block level, you'd have to loop through every state, which is not a problem. County-level stats can be gathered in a single call, IIRC.

The API seems to be the basis for the Census's new data platform: https://data.census.gov/