One of my all-time favorite CSGs has only bad or at best bittersweet endings, which probably tells you all you need to know about my perspective on this question.
More than fair. 
I understand that a lot of people don’t enjoy reading the story of a leader who struggles and fails early on, let alone play it with themselves in the protagonist role. That’s a 100% reasonable entertainment preference! I’m even more sympathetic to anyone who doesn’t like detailed management mechanics in their text-based games; the Rebels 1 winter survival mini-game is (on the kindest possible account) pretty close to the edge of what can be fun in text. I’m sure your discord has confirmed that you have plenty of mule-loathing company.
With all that said, it isn’t hard to survive Rebels. No pathway through the winter will kill you, and if in Ch 4 you pick the sane and reasonable path that almost everyone who’s not a fanatic or traitor-suspect is pushing you to pick (i.e. run and hide) you’re pretty likely to both survive and bring most of your band safely through to Game 2. But people understandably set their sights on other success criteria, like crushing the army sent to defeat you…and yep, aiming for that does set you up for failure.
My intent with Rebels has always been to have the failures lead to interesting places. Losing the Ch 4 battle and fleeing with Yed alone into the Xaos-lands will open up opportunities from G2 onward that a Ch 4 victor doesn’t get.
Of course Rebels does also have a fair few endings where you just die horribly. My genre expectations were shaped by Fighting Fantasy (for IF) and George RR Martin (for bleak fantasy) enough to guide me away from the typical CoG “no bad endings” guideline. And that, too, is something people can reasonably dislike. It’s a reason (along with its sheer length) that Rebels was one of the first games to code in chapter checkpoints, so that a loss can be remedied without a full reread.