Operating a switch mode power supply at its limits
Is it safe and power efficient to operate SMPS at their limits?


Yes, but not safe to operate them above their limits. Although an SMPS with appropriate testing should be able to handle a direct short.


  If this depends on the PS (maybe the IC) is there an easy way to know
  which PSs I can use this way and which not?


Check the datasheet, make sure it has the appropriate ETL certifications (UL ect). Buy from a reputible company (ie, a cheaper supply one area they usually save money on is the testing and components). All supplies that are tested to IEC standards are tested to the following (and a few more that I will not list):
1) Make sure that there is appropriate insulation or gaps for high voltage sections in the PCB, cables and components
2) Components that are above a certain voltage (mains voltage) have to be tested under additional ETL requirements
3) A fault won't shock the user
4) The supply can handle shorts at the rated current without overheating or destructing  


  If it's not safe / power efficient, why?


Power efficiency has nothing do do with saftey of the design. And at DC, the supply doesn't care what kind of load you place on it. If you have something that is switching on and off this can become a problem. IF you really want a safe supply then get a medical grade supply.