I can only guess, but, I take it you mean desktop pc type computers, or servers in metal cases that can be removed to access the internal items such as RAM, processor, HDD etc.
In these types of unit, except perhaps the very large, the only accessible voltages are ELV, and below that considere harmful, i.e. below 50V, and they are d.c.
Now, they tend to be +/-5, 12, & 15, so well below, and in general not considered hazardous.
There is however the low voltage incoming power supply, and, that is generally accepted as hazardous.
The actual power supply unit generating the d.c. tends, to be a self contained unit, encased in steel, to act as a Faraday cage, and, to ensure good earthing.
Thege generally have an IEC inlet, and are sealed units.
If, the guys are going inside the PSU modules, then that is a whole different story, however, if they are doing say, processor, and RAM replacements for example, then the hazards are very low.
I would expect that they have had some sort of formal "classroom" type training, such as a C&G course, or BTEC or the like, which should have covered the safety aspects, and that should be enough.
I would not mind seeing one or two of the team perhaps having done some training around the IET CoP for ISITEE, e.g. The C&G "PAT" course, or the like.
I feel that they should be ensuring the basic electrical safety of the units prior to return to customers, that is basically the Internal PSU, by example a "PAT" or similar to confirm leakage, IR, and earth continuity.
After all, the leakage and earthing will have a functional & reliability aspect as well as the safety one.