They're not? I mean, combined cycle plants have been built out the wazoo, so there's good data on that (replacing natural gas with hydrogen causes only minor changes, mostly in the combustor, and there are already industrial combustion turbines burning fuel gases that are mostly hydrogen.) Cost data on recent nuclear builds in the west are also widely available.

I suppose you may be meaning there's no good upper bound on how bad the expected unexpected cost overruns in nuclear can be.