The problem was; he also used very thick impasto-style brushstrokes. With Oil Paint, this often takes a long time to dry (and leaves a physical texture which will be visible if you overpaint that).
Some of Van Gogh's paintings are so thick, that underneath an outer layer of dried paint, there's still liquid paint trapped underneath, which may not dry for another hundred years.
But yeah, he was known for re-using canvases, as were many impressionist and post-impressionist school painters.
Will this internal drying of the liquid paint affect the looks of the painting in any way? ie will the painting look different in 100 years than it does today, assuming it is kept perfectly preserved and untouched in the ideal environment?
No, but it can lead to trouble when the painting is re-framed or cleaned because any kind of manipulation of the canvas can crack the paint. This is specialist work (especially on paintings with this kind of value) so they're in good hands but the problem is much less pronounced with thinner layered paintings.
Some of Van Gogh's paintings are so thick, that underneath an outer layer of dried paint, there's still liquid paint trapped underneath, which may not dry for another hundred years.
But yeah, he was known for re-using canvases, as were many impressionist and post-impressionist school painters.