Yeah, in hindsight, G1's end wasn't all that bad, especially when contrasted with the end of JTO. That being said, I recall it being unsatisfying for two major reasons.
Change in Central Conflict
Lack of Character Focus
Both of these can probably be attributed to the premature cancellation of the theme, which forced things to be shifted around and reorganized in order to deliver some semblance of a satisfying ending, but I digress.
The change in central conflict is one of the biggest problems with Journey's End; throughout all of BIONICLE, the goal of the theme was fairly straightforward. Defeat Makuta and awaken Mata Nui. Obviously, once Mata Nui was awakened in 2008 the main conflict became "Stop Makuta." The problem is that the story then continued onwards, introducing Bara Magna and all of the Glatorian, and suddenly the primary goal of the story began to resolve the tensions of an entirely new race of people, uncover the history of Spherus Magna, and reunite the shattered planet. This plot thread was introduced far too quickly to be effective, and it likely would have been far better if we had received the intended amount of years to develop the mystery.
The second problem, the lack of character focus, is a direct result of the change in conflict. Due to the story shifting location to Bara Magna in 2009, all of the characters we had grown to know and love throughout the prior 8 years of BIONICLE were put on the backburner in order to develop the Glatorian, Agori, and Mata Nui. So many new characters were introduced in 2009 that it makes it difficult for many fans to have any sort of attachment to them. In addition, many of the Bara Magna characters were instrumental in how the 2010 story played out, and took screentime away from characters who had been present in the BIONICLE story for much, much longer. Most noticeably, all of the Toa Mata besides Tahu and Takanuva are completely absent from the storyline, while characters like Gresh, Kiina, and Ackar get top billing. Finally, focusing on Mata Nui as a protagonist is fine, but when he has received development for such a small period of time in comparison to numerous other characters from BIONICLE's run, it becomes a problem, especially when he isn't exactly a very nuanced or complex character.
I think a better ending to G1 would have focused on the Toa Mata and Takanuva figuring out a way to destabilize or distract Makuta from the inside while Mata Nui fights him on the outside, rather than them leaving the Mata Nui Robot to fight Rahkshi on Bara Magna. That being said, then you run into the issue of introducing all these 2009 characters only for them to have little to do with the storyline's resolution, so really... I think it's safe to say that BIONICLE G1's ending was yet another victim of an early cancellation and leave it at that.