Special Effects Failure: The CGI and Flash animation would be impressive...in the late 1980's. Now, in the 2000s/2010s? Not so much, especially considering the leaps and bounds Adobe Flash animation alone has gone through (sometimes the best being barely able to distinguish between traditional 2D Animation). The CGI is also rather outdated.
Additionally, some art in the style of the old show was used for pictures of old characters (Such as Speed and Rex). To say the dissonance doesn't make it obviously look out of place is an understatement.
They Wasted a Perfectly Good Plot - Speed Sr. only makes himself known to his son at the last second of Season One, leaving viewers without an answer to why he went into hiding, what adventures he went through for 40 years, where the other original characters are, and what he's going to do now. Instead, we get episodes that recycle the same exact plot with the same villain in a school setting. Speed doesn't even get to race outside of the campus! Season Two doesn't seem to make things any easier to understand, as Speed Sr. leaves again at the end of the first episode, and only make sporadic appearances after that.
Later episodes of Season Two do flesh out the plot, among other things, and in fact goes into Cerberus Syndrome territory over time.