Are they clean enough to stay within the limits of regulations around pollution when run for long periods of time?

The generators I worked with were just massive diesel engines the size of a tractor trailer each. This was pre-def, pre-cat requirements. When I would switch on multiple load banks at once I could get a small puff of soot as the engines revved up. Provided the load was not varying heavily they would at least run clean enough that one could not see or smell any exhaust. This was in the 2000 time frame. Mind you the load tests are not real-world realistic at all and this can be further mitigated using step-start programmable PDU's in the data-center. Companies that own data-centers usually have a lot of political pull with the cities due to all the assorted taxes they pay, direct and indirect revenue and employees they bring in.

Cat has since added options for hydrogen [1] but I have no idea how many people have bought them.

[1] - https://www.cat.com/en_US/by-industry/electric-power/electri...