Basically it took everything that made Samurai Jack what it was and threw it in the bin.
In seasons 1-4, Samurai Jack was a plot-driven pulp serial about Jack finding his way home. GETTING BACK TO THE PAST WAS NEVER THE PURPOSE OF THE SHOW. It was just an excuse for Jack to go on his adventures and a way for the writers and animators to showcase whatever crazy setting they wanted. If you've ever watched Quantum Leap or the Incredible Hulk tv show you know what the deal is.
In season 5 all of the sudden Jack has to have an emo character arc and getting back to the past is The Point Of The Show. Which might have worked out if it had been executed better? But the pacing is so fucking weird and the character stuff is so underwritten and the finale is so fucking disappointing that it just makes me wonder why Genndy felt he needed to reinvent the wheel when literally all we wanted was more of what made the show great in the first place.
For my money the IDW comic did Samurai Jack far more justice than Season 5 did.