These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
1) Oracle has amazing powers and could have killed Robotnik in his own bedroom as shown in a flashback in the first episode.
2) Oracle instead gives Robotnik the heads up about the prophecy and that is what gets Robotnik to start looking for the trio.
3) If he hadn't directed the trio in the first episode to meet at a public place then their foster parents would have been safe and not turned into robots. So the Oracle is going with a 'I'll finish off whoever is left in the end.'
Alternatively, the Oracle is just a big jerk who likes to mess around with people.
Also that the Queen is a dead beat mom trying to stay as far away from her children as possible while preaching about 'responsibility' and 'not turning away from the past.'
There's a point where Sleet and Dingo stop being typical Saturday morning cartoon villains and become tired employees threatened into a job they don't want at all.
Maybe it's the mobin equivalent of biting the head off of a live bat.
Ear Worm: The opening theme. "Triplets born, the throne awaits, a seer warns of a deadly fate..."
Ensemble Darkhorse: Sleet and Dingo are surprisingly popular. Blame it on their fun voice acting or their Jesse and James like appeal.
Sonia and Manic (and possible Queen Aleena) are also this outside of Sonic Underground, due to sharing Sonic's design, while the rest of the characters just fall under Uncanny Valley.
Fan Nickname: Some people call an overpowered emerald fused Dingo "Chaos Dingo".
Harsher in Hindsight: Manic, in "Art of Destruction", complains to his siblings and the recently reformed ART, that the prophecy never said when they would win, and that it could be twenty years later. Given the Development Hell and eventual cancellation of the epilogue meant to be in Sonic Universe, it's more depressing if you were a fan of the show.
One of the early concepts for the first Sonic game when it was still in production was to have Sonic in a band and judging by the concept art images for it, it seems to be a rock band, though sources say they had to take that part of the game out to save space on the cart, and that Sega of America told their Japanese counterpart that that concept for Sonic wouldn't go over well with American audiences. Though it's not just the coincidence between that part of Sonic's origins back then in the games and this show that's funny but also the fact that the whole band concept was, like Sega of America predicted, received quite poorly by many who watched this show.
Manic, in "Art of Destruction", complains to his siblings and the recently reformed ART, that the prophecy never said when they would win, and that it could be twenty years later. With the Development Hell concerning the epilogue meant to be in Sonic Universe (which eventually led to its cancellation), this becomes hilarious if you hated the show.
Ho Yay: At the end of the second episode, Sonia kisses Sonic on the cheek for a job well done, and Sonic turns to Manic and says, "You're not gonna kiss me, are you?" Manic's answer? "No way, but you are kinda cute." One of the funniest lines on the show.
In another episode, Sonic asks for a dance while looking Sonia's direction. She accepts, but then he says he was talking to Manic, and the two begin slow dancing. The pair had a surprising amount of this.
Mary Sue: Sonic is so fast and destructive in many episodes that there's often no realistic need for his less-powerful siblings or the weaponized musical instruments. In fact, he could probably easily trash Robotnik's plans without them.
Moral Dissonance: In "The Do-Bot", members of the aristocracy are hypnotized by a robotic stylist at a hair salon, forced into admitting their negative thoughts concerning Robotnik, which are recorded and retrieved by Sleet and Dingo. Sonia finds this out and ends up giving refuge to the stylist robot. Sonic and Manic, however, want to let the robot get captured again, thinking that the aristocrats deserve what they get because they're rich, and were gossiping. Nevermind that the punishment for this is roboticization. They'd rather let dozens be given a Fate Worse Than Death before they find out Sonia was hypnotized into revealing where their secret base was, aka they don't care until it has something to do with them. The moral of the episode seems to be "Don't gossip", which makes you wonder where the writer's heads were.
Padding: In episode 6, the camera pans up to Dingo's face for seven seconds at one point.
The Scrappy: Sonia's vain, germaphobic personality and annoying voice place her squarely in this territory from the get-go.
Rescued from the Scrappy Heap: Subverted; Sonia's bout of amnesia makes her more competent than when she was with her brothers, as well as the fact that she seems to get more character development over Manic and Sonic.
Sonic and Manic, of course, due to Totally Radical personalities, though YMMV as well due to some character development.
And then there's those who think this of the music.
Strawman Has a Point: Manic, in "Art of Destruction", complains that no matter how far they go or what they do, Robotnik will just throw something else at them, and the situation doesn't ever seem to change no matter what they do. When you look at many of the previous episodes where the robots seemed to only escalate in how powerful they are, as well as the generally unchanging nature of the show, it's hard to not agree with Manic.
Uncanny Valley: Some of the characters in this show really crawl close to this territory. (i.e. Just look at Sonia's adoptive mother in the first episode-she looks like something pulled out of a Renaissance Fair!)
This version of Mobius has plenty of insectoids and bugs; some look monster-ish.