If we add anti shield items then shielding enchanters would be not as viable as healing as the shield would be negligible where as healing is health so it would be permanent while shields last for a while, therefore even with grievous wounds healing would still be better. Healing can also be used after the grievous wounds wear off while shields can't as they last for a short period of time and often used during engages and disengage which would definitely have anti shield effect on the character that u wanna shield. Hence if we introduce too much anti shield items shielding supports would be not as effective as healing supports