The main issues with just having it sitting on the coaming is it blocking your view and it sliding about if you don't have some method of securing them. Also remember that you'll need a battery pack if you aren't able to plug it into the aircraft.
In my case, both it and the portable battery pack fit quite neatly down the side between the coaming and the screen, so they are out of the way and also have a good view of the sky.
I've never had any particular problem with antenna placement, the actual limiting factor appears to be the GPS getting a fix. But that's no worse than on a tablet or a phone in my experience.