DMPS has a long safety history in Europe where it was first used.

DMPS is an easy scapegoat because it's not easily acquired and is expensive so many "alternative" doctors love to bash it.

Most DMPS "backfires" are related to the fact that many doctors do incredibly stupid things with it like use it on patients with mouths still full of amalgams, or inject gigantic dosages of it in a "push". You don't see this with DMSA since it's not an injectable. It only takes some searching to find people totally screwed up by taking too much DMSA for example- usually for the same reasons (large doses or mouths full of amalgams)

The other issue with it is that it's a sulfa-drug and a certain percentage of the population has a sulfa intolerance. The exact same issues arise with commonly used sulfa antibiotics.

DMPS still has the highest affinity for inorganic mercury of any chelator, so long as it's used safely (orally for example).
Even Cutler says it's his preferred chelator.

Of course, it all comes down to finding a good doctor. Many chelation docs swear by DMPS when used appropriately.

Anyone who wants to chelate should find a good doctor to monitor things.