" People usually have lunch around noon, so that means everywhere you go, you'll have to learn when lunch time is."
When the sun is highest in the sky is generally when people have lunch. Or, once you know that lunch is generally around the hours of 0300-0400, I don't think that would be difficult to remember.
"If I want to call someone in India from the US, I can check the time in their timezone and if it's something like 3AM, that's not a good time to setup a call, but if I don't have timezones, I know that 16:00 UTC works for me, but is that a good time for people in India?"
Finding out what time would be convenient to hold a call is something you generally do by negotating with the other party. If you're using global UTC, you can at least talk about the time in absolute terms, rather than having to do arithmetic to figure out how local time relates to remote time.