I had two full time jobs for eight months. The only issue was no leisure time

I've done this a number of times, last time 9 months.

Going on vacation for a week and starting a new job is very difficult to time. You come back give two weeks and have to work really hard transfering knowledge just when you need to be learning everything new job related.

Depends on the country. Here your notice time would be 1 month, so you can't just start "trying out" another full-time job during a holiday.

Edit: mistake in duration