Panoptes was a really cool boss that had the misfortune of being in an underwhelming DLC. His design was awesome and intimidating, and the fight was very cool thematically. People often ask for "single player raids". Panoptes was a single player raid boss. Add one or two more mechanics and he'd fit in with any other raid boss.
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From a storyline and lore perspective, yes, Panoptes was underrated. From a gameplay perspective, I get the hate. Personally, I had no issue with how that encounter went down, but I do understand where the backlash came from given the lack of interaction the player has with Panoptes, and vice versa. Same with Xol. As someone who never Raided, fighting Xol in a story mission/Strike was fun for me. But considering that Xol was a Worm God, I get why everyone else was so upset by the encounter. Lore wise, fighting Xol should have been a much bigger deal.