Placing people into Cryo-stasis has never been difficult… getting them out has been. The reason for this is essentially very simple – When they are frozen, they are killed. Thawing them out is not so much the process of simply warming them up as it is bringing an intact body back from the dead.
Human medical technology has continued to leap forward in the last three centuries, including a great number of techniques based around both life extension and healing. It is usual at this point for even the poorest of humans to get a gene-therapy treatment every decade or so, that cleans up broken cells and replaces many of them with freely manufactured copies, helping push back the copy-fatigue issue. Biological immortality has not yet been reached, but no one in medicine expects it to elude humanity until the end of the century. Currently, the life-expectancy of a human being born on a Terran Federation world or habitat is about two hundred years.
With this kind of replacement for dead cells, thawing people out of cryo-stasis is possible… but the entire process is still tricky. The body can still only take so much damage before it fails, so the thawing process has to be gentle, slow, and specifically tuned to an individual. Attempting to thaw a frozen subject without the aid of the computer that helped to freeze them has a minuscule success rate.
The most common complication to cryo-stasis is temporary amnesia – the process of rebuilding the brain’s damaged cells is by far the most difficult part of the process, and the one that the computer occasionally gets wrong. All of the pieces are still there, but the brain will need to remake some of the connections itself. This rarely leads to much of a change in personality, but can lead to an uncomfortable few days or weeks while it takes place. Out of the cargo-load of the Midgar-6, it is expected that perhaps 5 or 10 will have this issue.