Sub-slab duct systems are nearly always a problem and the only reason I use the word “nearly” is because I am holding out for finding ONE that did not have problems. I don’t think I have seen a one that either did not have water in them or have water stains showing that water had been in them. I have seen clay tiles used for these ducts, as in the photo. Even if the contractor wraps them in plastic, as I have seen, they still leak.
Further, leaks in the ducts can be a route for soil gases to enter the home, especially if the duct is a return. If the area has radon problems, such soil gases can be a serious problem. In that case, I am thinking that the only resolution would be to install new duct work.
Now, maybe someone has observed a subslab system that doesn’t have problems, and maybe if the home is built high enough above grade, it might not at least have leak problems. But, I see no way to avoid soil gas migration problems, unless the fan is run constantly and the ducts are on the supply side.