To give you a brief background on the sonotube, when the house was built, before the slab was pored, the proper way to setup sonotube was on concrete blocks, elevating it off of the ground (where those friendly, cardboard loving critters live) then pour concrete over the sonotube, encasing it - so you essentially have a tube made of concrete lined by cardboard. Problems occured when the builder skipped or missed the concrete block part of this process. There are other issues that sometimes occcur if water gets into the sonotube but all issues are likely to be discovered by a duct scope.
To summarize, I would NEVER buy a house with in-slab duct work without having a duct scope, and I would personally purchase a home with in-slab duct work without a second thought if it passes inspection.
If a home has Sonotubes I would definitely recommended that a buyer have the ducts scoped. Everything I have ever been told by HVAC experts is that itâ€™s a matter of when, not if they will fail. But with said, I have seen 30 year old homes that have no problems. If the ducts are compromised, the only economical solution I have seen is to install a new system in the attic (if available), or on the roof if flat.
I hope it works out for you - Tego