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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I think it's pretty obvious why it was nerfed, it used to grant enough damage to one shot body shot with no issues, give enough health to survive almost any super in the game (could even survive a point blank Rocket), you had a mini tether for a grenade, could lock down flags or zones without needing to worry about anything and when you had 5 defenders on the field your team could get unlimited supers (which worked well with Golden Gun). It's not even bad now at all, try using it in Trials of the Nine... probably one of the best supers to use in something like Countdown.