How would summoning demons/devils fit into this boon, or the summoning of more powerful creatures? Even if invoked on powerlevel 9 the hit points are very low and the limit to three attributes does not reflect the potential of more powerful creatures. Or is Summon Creature not intended to summon more potent creatures?
There are two ways to go about getting more powerful creatures: the animation boon, or the companion feat. The animation boon doesn’t really allow for supernatural creatures, but GM fiat can allow it. The companion feat allows you to build a fairly powerful ally.