Street Wyrd states ally spirits can perform bound and unbound services an unlimited number of times, and lists sustaining a spell for (force) rounds as a bound service.
This feels somewhat abusable to me - make a force 1 ally spirit for 8 karma, and have it sustain any number of spells for you. It suffers a dice pool penalty far in excess of any of its dice pools, but you can just not have it do things.
I can see some pretty easy interpretations/house rules to reduce this - if it requires an explicit command each (force) rounds, or if you can only have an ally spirit sustain (force) spells, or if excessive sustaining starts to be counted as spell binding, which would immediately kill your force 1 ally spirit by reducing it to force 0. Saying that asking an ally spirit to do this is mistreatment sort of helps, but a force 1 ally spirit probably can't do anything about it. There might be reason to think an enemy could track the sustained spells from you to your ally spirit, then kill your ally spirit (which is force 1 and suffering a giant penalty to all actions)
Even with max (force spells), being able to spend 8x karma to sustain x spells seems very strong - focused concentration is 12 karma per spell, max 3, quickening is 1 karma per spell, and a sustaining focus is 2*force per spell. They all have different details so it's not wrong to have different costs (though quickening is probably way too low). Maybe (force/2) is a better limit?
I feel like "ally spirits are good at helping you sustain spells" is an ok place for ally spirits to end up, but "ally spirits allow you to sustain infinite spells for 8 karma" is not a great place
