People were shocked that everyone died and that he never bothered to beat future Aku.
Meaning all those folks that showed up in the ending battle were doing it Trunks-style, to ensure there was at least one reality where the evil was destroyed. This sort of thing makes sense when you don't spend any time in that world, but not when you've gone on 4 seasons of adventures in it prior.
Hell people were salty as fuck Future Trunks' reality was disintegrated in its entirety just to protect the main one, and that was a drop in the bucket of Dragon Ball. Now take that and amplify it by the entire show, and you get the bitterness over Samurai Jack's ending.