I have not heard of people that had a server not running after an upgrade. Most of the issues with the latest updates were cosmetic issues (with the server-manager navigation panel being disorganized, but not being disfunctional, others are concerning badly coded and tested contribs).
If you have a stock server and have issues with upgrades please report them in the bugtracker so the devs can device a fix, if it is a general issue or help you solve your problem if it is an individual case.
Updates to the SME Server core are tested by the dev team, for stcok SME Servers only. If you do not have contribs installed you will not have much to worry about.
One of the problems on this is that only problems are posted, as people are looking for a solution. Succesful installs rarely get posted as there is nothing to fix 
Most of the problems caused on updates are because of user errors AFAIK.
Thank you for your reply, the reply you gave me is the political and ethical correct and the best answer,
But your answer lets-me concerned, very concerned, because of 2 (two) reasons:
First, it is because you have not answered what I asked.
Second, because I have a friend of mine, who tried to do a yum update on a server version 7.2, which had never been updated, just installed from the CD (downloaded). This friend, posted on this forum it’s problem, he did and answered all the questions the dev. team and the doc. team asked for, bad got to the end with being unable to update the server, and no solution for the problem.
Fortunately because a previous bad experience, of this friend, he (tried to) made de update on a test server. Now because de things went wrong on the test server, and because the lack of solutions, he is afraid on doing the update on the production server.
...And I am afraid too!!!!
My friend was told to: open a bug, but, because my friend is new to SME (but not new to Unix) we got crazy wit this kind of answer. We considers himself new, and not with expertise to open bugs. Should People open bugs indiscriminately, when it can be a user problem? It is not more correct, to be the experts to open the bugs, if they have no solution, and are sure that it is a true bug...???
Regards.