The sarcophagus is not an effective permanent enclosure for the destroyed reactor. Its hasty construction, in many cases conducted remotely with industrial robots, means it is aging badly, and if it collapses, another cloud of radioactive dust could be released. The sarcophagus is so badly damaged that a small earth tremor or severe winds could cause the roof to collapse. A number of plans have been discussed for building a more permanent enclosure. Most of the money donated by foreign countries and contributed by Ukraine has been squandered by inefficient distribution of construction contracts and overall management, or simply stolen.
About 95% of the fuel (about 180 tonnes) in the reactor at the time of the accident remains inside the shelter, with a total radioactivity of nearly 18 million curies. The radioactive material consists of core fragments, dust, and lava-like "fuel-containing materials" (FCM) that flowed through the wrecked reactor building before hardening into a ceramic form. By conservative estimates, there is at least four tons of radioactive dust inside the shelter.
Water continues to leak into the shelter, spreading radioactive materials throughout the wrecked reactor building and into the surrounding groundwater. The high humidity inside the shelter continues to erode the concrete and steel of the sarcophagus.
I guess this is one more thing we need to worry about. Considering the disastrous effect of just a tiny portion of the fuel, I find it hard to imagine what would happen if the sarcophagus was breached (and I haven't seen any informed speculation on this). It really puts our other everyday concerns in perspective... for a while at least.