Sounds like it's different with a car.I have been using a oil extractor for a few years now and when I started using it I extracted the oil and then removed the oil drain plug and so far on everything I have used it on nothing has come out of the sump with drain plug removed. It gets everything out of the sump that a drain plug removal does and maybe more.
I use on the mower, golf cart, generator and power washer. Works on my cars and trucks also.
I have heard a few high end auto manufactures don't even have drain plugs on their vehicles anymore and require a extractor to do oil changes.
Don't know anything about using on a boat as motor angle and the way a boat sits on the trailer and the sump design all play into how much oil you can get out with the extractor?
No trailer on my 33' or 37', changed them both while they were sitting in the slip. Always had about a quart remaining. Same with my 24' though, which I did on the trailer.
On the generator and riding mower it's too easy not to just remove the plug and let it drain out. On the Ram I just let the truck lift itself up.