Vulkan: Uniform Buffers versus Push Constants for static data
UBOs can be updated at any time with vkCmdUpdateBuffer 


From the specification: "vkCmdUpdateBuffer is only allowed outside of a render pass." So "at any time" is not the case.

Even if it were allowed inside of a render pass, it's still a transfer operation. Which means you need to synchronize the memory transfer with the commands that use it. Which slows down performance.

For the general Push Constant vs. Uniform thing, use your judgment. By "judgment", I mean just look at how they work. Push Constants allow you to change their data at any time without doing heavy-weight processes like memory operations, synchronization, or altering the descriptor state. Clearly, they are for frequently changing data. How frequently is "frequently"? Well, that's a judgment call.

Failing that, profile the performance difference.