If you go into Sonic Underground expecting SatAM Season 3, you will most likely be disappointed. Although some elements from SatAM do make their way into Underground (such as the "resistance vs dictatorship" concept), none of the SatAM characters appear except for Uncle Chuck in one episode. The new cast is primarily Sonic and his long-lost siblings Sonia and Manic. I saw Underground before SatAM, and I enjoyed it a lot at the time. For a show that I had next to no expectations for, I found it surprisingly good, mainly because I liked the three main characters. However, I haven't watched a single episode in at least 3 years now (time sure flies), so I'm not sure if my views will change when I re-watch the series.
Personally, I'd say to try watching Sonic Underground's first two or three episodes. If the series doesn't click for you then, then I doubt future episodes will change your mind (although we do get to see knuckles for four episodes). From my experience, Sonic Underground is a show that Sonic fans will either like or dislike depending on a lot of variables.
I hope this post was helpful to you Lynchenberg. Have a great day.
