I wouldnโt say Timeless is a great show.
Relies too much on tropes and other narrative shortcuts; doesnโt focus on characters in the first episode. Lot of lazy writing.
But thatโs not why people hate it, of those who do. No, thatโs because the main character is a woman, many of the other castmembers are women or black, and they are truly foregrounded in the narrative. And itโs a โmainstreamโ show, not one that is targeted at another demographic.
Hereโs a capture of all the main cast:

Wonderful diversity. The white guy is the token.
As movies worsen, TV gets better. My partner and I were just talking about that yesterday. These days, mediocre TV shows usually have higher quality than all but the best movies. True of Timeless, too.
For all its faults, the first episode of the show had a nice twist at the end that I did not anticipate. The show it turns out wasnโt quite as spineless as I thought at first.