Unless I'm mistaken; In custom games at the moment you can only view your own victory conditions and taboos? Should you be able to see your opponents?

It would change the dynamic if so, it's a different game if you know that your opponent has to kill you with an elf/collect x number of mana sprites/only survive/has a taboo on mounts, than if you're not aware of those settings.

Maybe an idea situation is to be able to toggle on and off whether those conditions are viewable?

No cuz I can open up a whole new possibility for cheese play. Example if you know your opponent has to kill using a specific monster you're going to prioritize that monster more than anything else; after all if he kills you with anything else they lose the bonus. Or you can steal sprites even when doing so would be more or less useless just to deny your opponent.

Taboos are less exploitive on the surface but just as bad. Example of taboo on summoning will encourage your opponent to spam monsters so they can overwhelm you

I know what you mean regardling things like kill with x, and for a game like that if each player is given a different kill with creature then you may well want to keep it hidden.
On the other hand, if you giving all players the same kill with creature then it could also work to let every player see that it is the same for all of them.
Not seeing the creature adds suspense, seeing what it is alters tactics.

I think it can more the case to have an open system with mana collect if each player/team has the same objective. I.e. There's 5 sprites on the map, and each player or team is tasked with retrieving 3 of them. I think in that case it's better to see each other's objective.

I think where you may be more likely to want objectives to be hidden is where player A has 'kill player B', and player B has 'kill player C' and so on.

I think you'd also be more likely to want hidden objectives if they vary for different players. I.e. Player A has 'kill with Elf' and player B has 'Collect 3 sprites.'