I've used all 3 to ship and received from all 3. So far, I've not had any shipping issues, at all.
I only use USPS, if USPS offers overnight, to the area I'm shipping to, OR if there's a post office, within reasonable distance of the receiver, for Hold for Pick Up. As an example, I recently shipped corals from Spokane, to Southern Idaho. The zipcode of the buyer, did not have Over Night available. However, another post office, which was only 12 miles away, did offer Over Night, so we went with that post office and Hold for Pick Up.
To address the "liquid" issue: I've never ran into that issue, at least not as a problem. I've always told USPS what is in the box. They've never questioned anything, only been interested in the fact that I have corals...lol. I've always been honest with them. They know there's saltwater in the package.