Mutual intelligibility is problematic for languages with a dialect continuum.
Native speakers that live next to each other can understand each other without a problem but the farther apart the regions are the more likely it is that they can't anymore.
For example German. The Swiss Germans and the South Germans can understand each other but if you let somebody from the north of Germany speak with them in the local dialect, they won't.
Linguists usually don't even classify languages as dialects anymore. It's almost always a political minefield.
In biology they would call that a "ring species". Sometimes two nearby populations can interbreed, which means they are the same species. But distant populations are so different they can't interbreed, and so must be different species. But figuring out exactly where to draw the line is problematic.
Native speakers that live next to each other can understand each other without a problem but the farther apart the regions are the more likely it is that they can't anymore.
For example German. The Swiss Germans and the South Germans can understand each other but if you let somebody from the north of Germany speak with them in the local dialect, they won't.
Linguists usually don't even classify languages as dialects anymore. It's almost always a political minefield.