I always get

Warning: IKK too small - model effect disabled!

in ngspice with transistor sims. Can that be safely ignored? I'm on open_pdks 1.0.470

I have been ignoring it. . . which is not really an answer. There are a handful of parameters that ngspice doesn't handle or handles differently than other simulators, and I generally assume that they are higher-order effects; but it's probably a good idea to get a list of them and make sure the community has an understanding of them.