Unfortunately, there’s no really clear answer to that question. To be honest, I don’t how much Ubuntu is writing to the installed drive, though my guess is probably not a lot. As long as you’re not memory constrained on the machine that you’re using it on, there’s a good chance that it’s mostly reading. When you become memory constrained, Ubuntu may fire up a paging file, which could cause a lot of write activity. But my guess is that Ubuntu itself isn’t doing a whole lot of writing.
I am a bit concerned about the flash drive, but it actually makes sense. Let me explain.