I originally used rivets but then changed to liquid metal. Never had problems with either really, rivets will still cause a bit of a restriction though whereas with liquid metal you get a flat smooth surface.
Using liquid metal, make sure you put a good chamfer on both side of each hole (so the cross section of them would be egg timer shaped!) or else the stuff will just slide out when it's set.
Rivets fail all the time when they are subjected to vibration and movement. I remember one person posting once (years ago, might have been the old PSD forums) saying that a rivet had fallen out and caused damage to the pistons.