I've used a biscuit jointer for edging several times, and I find that the slightest slip or shake of the hand translates into the workpiece. As a result, I would say I get a perfectly aligned edge only about 60% of the time, and the other times I have to compensate somehow (either by sanding the edge flat, or removing an offending buscuit and using clamps to align).
I have never once had a problem with splining though. From my very first attempt, I have gotten perfect results every time. I use an interchangable arbor with a 1/4" cutter, so my piece slides flat on the router table (or I slide my router on the surface for really big pieces). The router always slides on the 'good' surface, which means the grove is the exact same distance from the good surface on both pieces, and as long as you use tight splines, the two pieces will allign perfectly with 0 error. The only way you would get missaligned pieces using this method is if the router leaves the surface of your piece, your piece leaves the surface of the router table, or the depth of the bit changes -- none of which are likely to happen.