I've fairly new to all of this, but I've researched the manifolds fairly heavily for my upcoming install. My input is that the actuators seem to be specialized for that particular brand and type of manifold, which makes me nervous for finding replacement parts in the future. The moving parts seem to be the weakest link, so one could reasonably expect failures before the static components. If you're already planning on breaking up the zones onto separate manifolds, why not zone valves or separate circulators before each manifold? The popular zone valves all seem to have been around for a while with replacement power heads available, and if not available, it seems easier to substitute a new zone valve vs. having to figure out a manifold actuator replacement.
Also, I believe I have read somewhere that increasing zones (3 zones to 8 zones) may decrease your efficiency (many more zone calls not happening at the same time). It would certainly increase the cost of parts as well!