From my POV, it was mostly a product & design leadership failure. Ironic, because the team had phenomenal design and product work individually. They took a cheap way out with building a second browser, lost most of their userbase, and in doing so, admitted that users didn't want what they were building the way they were building it. It wasn't magical or intuitive, or else it would've been fine to build into Arc.
But instead of doing the hard overarching product & design work and making the tough calls, to build that roadmap step-by-step and add the magic, they decided they couldn't reconcile these two ideas into one, and needed two browsers.
One anecdote: Despite sharing mutual investors and connections with them, I never even heard back on any feature ideas or bug reports sent to the founder.