Flemish is more of a political construct than linguistic, it's a grouping of belgian-dutch the coastal, brabant and limburg language groups with each having their own regional dialects.
Now, being from Belgium, even within that small part of the country where everybody is supposed to speak Dutch, I genuinely don't understand people from near the coast, which was about 150 miles from where I used to live.
What I find interesting is that the differences in Flemish dialects make them much more distinct than what would normally call dialects. There are significant grammatical difference beyond the usual vocabulary differences. For instance, coastal Flemish conjugates yes and no[1], Limburgisch is a tonal language.