Here are the definitive statements:
- The NUC8i7HVK, NUC8i7HNK, NUC8i# CY*, NUC7CJY* and NUC7PJY* NUC products have native HDMI 2.0a support.
- The NUC7i# DN* and NUC7i# BN* NUC products utilize LP-Pcon ICs that support HDMI 2.0a.
- The NUC6CAY* and NUC6i7KYK NUC products utilize LP-Pcon ICs that support HDMI 2.0.
- All other (older) NUC Products utilize LP-Pcon ICs that support HDMI 1.4a.
I am not sure what you mean by "safe". There are a few cases (the NUC products in item 3) where an upgrade to the LS-Pcon IC's firmware must be installed before compatibility is guaranteed. Other than that, the outstanding issues have to do with the vagaries of the Intel HD Graphics drivers.
Hope this answers it,