In the beginning of Rebels, the show played out pretty well, small time band of rebels(who weren't even in the Rebellion as a whole) fought against the Empire and the stakes were fairly stacked against them and did lose skirmishes just as easily as won them. As well as have original characters on the Imperial side. Then, during the ending of season one they killed off most of the imperial cast to bring in Tarkin and Vader. It was alright, but here's where the problems for Rebels came in:
They relied way too much on cameos and plot armor for the Ghost Crew. After season one, there were no stakes for Kanan and the others(except for the last season, but it was too late for me to really care anymore). The Ghost crew just started winning every battle to the point of where you could guess what happened by the next episodes' previews. The writers introduced us to Phoenix Squad(red shirts for the good guys, basically) but then forgot they existed until a plot needed resolving.
Pretty much, the writers created a too OP as fuck Rebel crew and made them powerful by making everyone else retarded. The cameos were annoying as shit because they'd be used for an episode, make a quip or two and then they'd be gone. The problem with that was there were hardly any interesting original characters and the original characters that were showcased had no day in the limelight and were usually killed off by a cameo appearance to "Raise the Stakes".