In fact the Q's immortality was a key argument in Quinn's right-to-die case on Voyager. Q, despite being dispatched by the Continuum to argue against him, revealed in the end that he was an admirer of Quinn and his renegade philosophies, which led to him having a child with the Q played by Susie Plakson. I would say it's likely that Q could also be influenced by the idea that mortality makes life and the time you have left all the more precious, so he is part of a Quinn Faction of the Q who choose to give themselves a finite lifespan. Now that could have been a billion years from now and he could still come back to be with his bestie JLP on his dying day.
Q seemed pretty afraid of Guinan, and she certainly gave the impression that she could do him some damage, even with his powers. It might have been just fear for the chaos it would cause with "lesser" species, but Q was adament that his son follow the Continuum's rule about not provoking the Borg, which could indicate some fear. During the Q Civil War, their fatal "shots" at each other in the Continuum manifested in the form of supernovas in the real world, so it's conceivable that if you could trigger a supernova (with trilithium, Tox Uthat, protomatter, etc.) while a Q was in proximity that might wound them. Other godlike beings like Wesley and the Traveller or Kevin Uxbridge or Nagillum or the many godlike beings in TOS might have a chance. And of course turning the Q against each other is always a possibility, as they seem eager to punish outliers of their kind for breaking established norms, and will either depower them and leave them to fend for themselves against the beings they have tornented, or they will exile them into a comet, or they'll send a tornado to destroy one specific home with two depowered Q (but miss noticing their daughter, so not omnipotent). Voyager was shown how to enter the Continuum by the woman Q, so the means to establish communications with them instead of just waiting for one to appear exists. Just no one wants to deal with the Q, so they don't mess with it. Plus, it was very difficult to do, even with her help.
The Borg would also show that they can punch into other dimensions, spaces etc on the regular and would incorporate technological developments into their own immediately and fleet wide. I suspect the Continuum is afraid of the Borg, as really, while the Borg could absorb truly enormous losses (Their losses during the five month Species 8472 war, hundreds of planets over 300 Cubes and well over 4,000,000 drones) they'd barely been scratched. Facing sheer numbers, the Q only need a single Q to be lost to the collective and assimilated for us all to lose. The Q had managed to absorb the entire knowledge of the universe, debated everything, apparently had technology that caused stars to explode from inside their little pocket dimension. The thought of a Q-Powered Borg us utterly terrifying. We already know our "favourite" Q has a soft spot for Humanity despite his occassional spitting vitriol. So he might even be one to advocate "Dont Mess with the Borg."
Is it a failure? Well, if your season hinges on dredging up and facing the worst sort of childhood trauma but is almost totally unengaging emotionally, that might be something of a failure. But Brent Spiner was kinda funny. And Whil Weeton gave us a minute of dazzling brilliance. Also Rios and his lady are sexy, sexy people with corazones de oro.
And that scene when Q slapped Picard, that was actorly acting with a capital A by actors' actors. "This is not a lesson, this is a penance, blockhead!" I still get goose pimples.
Small little scenes like that were actually pretty great. Some of Q's motivation though bugs me. Why try to put Rene off going on the Europa mission?
It's a bit convoluted maybe but my take was that he knew what exactly what course of events he had to take that would lead to Picard letting go of his past. That included him sabotaging the Europa mission so that the confederacy timeline would exist and Picard would go back in time and help Renee.