What was the worst part ?

None. Tests were sometimes a little annoying, but not painful.
I did: upper GI, sleep study, numerous blood tests (one came back high thyroid which created the need for more tests).

Nothing was really uncomfortable, the upper GI is a little gross, (barium swallow) but totally tolerable.

I wouldn't think too much about them, show up when you need to and know you're working with pros who have done this tens of thousands of times, and go in knowing nothing will be terrible.

Worst part is all the appointments / scheduling !