Not being a user of agents, my question is 'does the agent always find the root cause and remove the defect'?

The way I got good at finding the hard ones was finding hard ones. Without that practice, I would be helpless.

Reason why it's super hard for juniors at the moment. For "agents" to produce good results, the operator must have the experience to direct it in the right direction and properly validate the output.

I've had some weird bug investigations where it has suggested wrong fixes confidently multiple times before arriving at the actual fix. Only with extra details from manual testing did we arrive at the real bug. The ~5 first suggested fixes would've hidden the root cause even more.

I've found agents very effective at both idebtifying root causes from vague descriptions, and explaining both the problem and the fix to me.

It's not a panacea though. Sometimes it comes up with bad causes and/or solutions.