I can only speak for the one video of theirs that I've seen, which is the one on "Greek Armies during the Persian Invasions". This video is now more than 2 years old so it may no longer be representative of their channel.

That said, the video is a catastrophe. It's probably the worst video on this subject that has appeared with a popular history channel on Youtube. Nearly every sentence contains false or misleading claims. Every image is wrong. Every map contains blatant errors and anachronisms (towns that didn't exist yet, were of no significance, or whose names are spelled wrong).

No doubt a big part of the reason this video is so bad is that its source base is 50 years out of date. You can see the works they consulted; with the sole exception of Tim Everson's book on Greek armour, every single piece of scholarship they read dates from the 1960s or 1970s. I can forgive them if it was the best they could get their hands on, but it goes without saying that no scholar can take this seriously - especially given that our understanding of Greek warfare has changed dramatically in the last 20 years. As an attempt to inform people about a historical subject, a video based on this reading list is DOA. And that's saying nothing about the errors introduced by the makers' own limited understanding of the period.

It is quite possible that they do better in fields that are closer to their hearts and their own research interests. But certainly when it comes to Greek antiquity, I would urge people in the strongest possible terms not to watch Kings & Generals. You can literally pick up any book from the last decade or two and know more about the subject than they do.

Some people pointed out mistakes and inaccuracies from this channel in r/badhistory and for my part, I think they made not-so good videos about the Gallic Wars. They are lacking a critical approach on the source's material, they are excessively relying on Rome 2 Total War notably to locate the tribes on the map (which is generally wrong) and they are not studying the locations they are portraying for battles and encounters. For example Geneva has been put on the wrong side of the river.

Furthermore, the channel is popularizing common misconceptions, outdated theories and myths uncritically. For example, the Marian reforms (which probably wasn't a real reform) or even worse the idea of a lost legion of Carrhae in China with the story of the fishscale formation at Zhizhi.

The channel has chosen quantity over quality.