I'm no good at writing lengthy arguments so I'll go with these points:
30 seconds as opposed to 45 seconds (optional but usually always chosen by many in D1)
Can be broken by conventional weapon fire
Only supplies armor of light, blessing and weapons of light have been removed
Cannot generate more orbs as it is continually damaged
Death usually accompanies a nova bomb that was thrown from outside onto the shield, which makes no sense
The only thing that was improved is the deployment time and animation, everything else was shit on for some reason.
And yes I know that we 'have two supers in one', but defender has been gutted. At least make it so it can't be destroyed by gunfire.
Sentinel is also way more useful in blocking damage as you can actually move around while doing so, but sometimes you gotta protect a zone or your allies, rather than just yourself.
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You're lucky you even have the old super back. The real question: why were the old subclasses taken out? But that said, I'm not sure why they brought it back if it's not worth using. I feel like having another perk in its place making sentinel better would be the smarter choice.