The biggest risk with a passive like that is that it would risk making him a lot less reliable, when the intention behind its design is to give Malzahar very reliable initiation. The advantage to the current passive is that it can also be deactivated with a basic attack, whereas a spell shield could risk giving some champions an overly hard time by forcing them to waste a spell.
With the kit he has now, Malzahar is probably fine even when he reliably ults people and ignores incoming damage for a bit. The problem with him right now, imo, is that he deals too much damage. I'd support reducing the duration of the null zone he spawns with his ult down to the duration of the channel (i.e. it stops when he stops channeling), and possibly nerfing the damage on his W or E (his E should probably also have its range reduced to his autoattack range), but I'd keep his resilience as is, at least for the time being.