As a non native speaker I may be wrong in saying this, but my best friend is Flemish and he did explain this to me:
The Dutch often use the r at the back of the throat, but they are use the "tongue" r. I believe it's an accent thing, but I think they also pick and choose for different words. For words such as "werk" when there is a single rolled r (a tap), it's far easier to do with with the tongue, but for words like "graag" then I would go for the g at the back of the throat and then move it forward to the back of the throat r.
He also said that in Flemish the r at the back of the throat is moved forward towards the bottom of your tongue. To me it feels a bit like trying to roll and r and spitting at the same time - it's a little bit disgusting haha.
Hope this was of at least some use!