How reliable are npc allies in coming to your aid? Will they just faff about while youre being wasted?
They're fanatical about helping, which is why they can seem so random.
From what I've seen, the AI really doesn't like leaving ships in reserve; given time, they'll throw everything they've got into whatever war they're currently fighting, regardless of whether the additional fleet power will make any significant difference. The changes to Warscore also mean they're reluctant to surrender even when the vast majority of their fleet lies in ruins and their spaceports are all wrecked, so long as they haven't lost many planets yet.
These two factors act in combination to make the actual tonnage an ostensibly committed empire will send to defend you (or attack on your behalf) really variable. Your vassals and/or space liege will send their entire fleet your way, but they may well have all their ships halfway across the galaxy fighting some tiny two-planet empire if not destroyed outright.
I've found that vassals, at least, will supply troops more reliably than they will warships, since they don't need their spaceport infrastructure to create transports
en masse and those transports will spawn with their best technology automatically. If you bombard planets down and leave a small force to keep them down, you'll see vassals coming to take them with some reliability.