You might not have seen but a good server power supply supplies enough current to the parts to ensure smooth operation. Reliability will greatly reduce downtime. The power supply generates a lot of heat which is not healthy to a server. Thus, manufacturers introduce fans to reduce the heat to prolong life span.
Some use 2 fans and some use bigger fans due to its size. Hence, it is different for a 1U server and 2U server power supplies. Size is not the crucial factor to determine how fast you can bring down the temperature but CFM. CFM stands for cubic per feet, the airflow is the crucial factor to reduce the heat. Usually, the higher CFM spins faster, hence more noise is generated but not the size matter.
There are brushless and ball bearing fan. The later lasts longer, better MTBF compared to a brushless fan. Of course, they are many other components like capacitors are crucial too. So, if the power supply is cheaper than usual, either it is an imitation of a certain brand, pre-owned or its components’ price are lower. These are the power supplies you should avoid.