IMO, flagships do not suck, because this statement is an exaggerated generalization.
What sucks is overpriced smartphones that come with the flagship tag with 500+ dollars/euros price difference above the mid/high end devices and offer only 10-20% performance increase above the mid/high category, one or two gimmick features and edgier design. Yes this sucks.
But there are a few great flagships out there, that 1) justify their premium price tag (only pixel 2 lineup imo) and 2) do not overcharge you because of the flagship tag (oneplus 6, 5, 5T, Nokia 8, Mi Mix 2s, Mi 8, Lg G7) and offer you great performance without compromising almost on any category in comparison to their Overpriced Major League players flagship devices.
Also a lot of mid/high end devices do not make any sense. For example Tech Altar is praising his Nokia 7. But for me this is a choise that makes no sense. That is because in the price range of the 7 plus you can opt for the Nokia 8, which is both cheaper (around 30-50 bucks/ euros bellow), faster (sd 835), has ois, better screen (1440 x 2560) and better materials (anodized aluminum back vs plastic back with ceramic feel coating). Only thing you miss from 7 plus is a bit battery capacity, that rounded corners design and 18:9 aspect ratio but design is objective.
And as linus says, a new garbage is still garbage but an older diamond is still a diamond, so a man can opt for an older flagship rather than a newer midranger and for the same or sometimes better price.
So yeah, statements like this are exaggerated generalizations that help little to no the average consumer.
The smartphone market is multilayered and diverse with options for any pocket, preference and need of use. There is no one size fits it all advice for purchase.