Hard work always ran at the core of Dragon Ball Z's appeal. Inspiring people to get into shape & persevere in the face of unspeakable odds. Perhaps this is the most appropriate place to focus our critique.
What is Dragon Ball Z?
It's impossible to speak for everyone when it comes to what message you take away from the show. However I would like to make a case, that perhaps some of the backlash this series has received, can inform why people acted so negatively towards it (at least in comparison to DBZ).
"Goku never wins"
I can understand both sides of this argument.
Lets first argue against this sentiment.
Beerus: Beerus is not really a 'bad guy' he acts as a sign of whats to come for the story. After obtaining a new transformation Goku ISN'T able to beat Beerus. Which I find personally fascinating because that usually doesn't happen in the show.
Frieza: Goku technically wins this fight, but is cheated out. But in a certain perspective this was a good decision. Goku's first battle against Frieza is ICONIC, but it only turned out that way because there was a entire arc of buildup. Vegeta has progressed tremendously as a character and his battle with Frieza holds more weight. However Goku does kill Frieza last minute, robbing the battle of a proper dramatic ending.
Hit: He plays a similar role to Beerus, but on a much smaller scale.
Zamasu: This was the most traditional Dragon Ball Z arc in the whole show. But much like the Android Saga, it ends with Trunks victory. Remember Gohan Vs Cell wasn't the end of that arc. It was Trunks vs First Form Cell when he returned back home. Goku killing Zamasu doesn't hold the same weight because it's not Goku's world thats in danger. It's Trunks.
Jiren: Disappointing end boss. Not much else to say. (AWESOME fight though)
It can easily be explained why certain decisions were made in the story, but the major complaint runs back to what I spoke about before. Hard work.
In Dragon Ball Z, when Goku beat the final antagonist, it was a thematic representation of hard work. That if you keep pushing yourself, you can surpass unspeakable odds. But with the lack of final boss defeats in Dragon ball Super, that theme isn't as resonant. Goku progresses but it never feels like his training meant anything if he still can't win in the end.
Final boss fights matter in DBZ, they act as that big moment that makes the whole experience worth watching. But with no end goal for our heroes (not to mention people like Whis and the other angels still rank HIGH above the Z fighters) the whole workout is just draining.
Dragon Ball Super is not a BAD show, but I understand why people were upset. I still really enjoyed myself despite this above average score, and I would recommend watching the last couple arcs after the movie re-caps.