I bought a fully cooked pack of frozen sausages. On the instructions it says to eat cold(Defrost only in the microwave) and to eat hot(boil them for 2-3 minutes). Why can't I heat them in the microwave, if I want to eat hot? Is it for quality or safety? Would it be safe to eat if reheated in a microwave?
I freeze cooked sausages and reheat them in the microwave, so it can be done. I prefer to defrost in the fridge, but if that's not and option, the key is to go gently and rearrange. Give them a couple of minutes on medium power, move them around on the plate, then a couple more minutes. When they're soft throughout and warm at least in places, give them 30s on full, check, and maybe another 30s (having moved them).
This assumes 2-4 sausages, say 150-200g or up to half a pound. The reason for moving them around is that the microwave will have hot spots and cold spots. The rotating turntable helps, but there are often concentric circles of more- and less-heated areas.
The instructions that claim you can defrost in the microwave to have a cold product are nonsense anyway. It won't work. At that point I'd trust my experience over the rest of the instructions as well.
A microwave oven's heating is selective. It heats liquid water best. As a result, water tends to boil out of the food while heating, particularly if it starts out frozen and is cooked quickly (the frozen water doesn't heat as fast, so the outside has been boiling off for a while by the time the inside is up to temperature).
So if you fully microwave a sausage from frozen at full power, I'd expect the result to be somewhat leathery and wrinkly. I suspect that this is the motivation behind the instructions: not around safety, but around aesthetics.
You can almost certainly get good results, though. I would suggest microwaving at low power, moving them around as Chris recommended, and/or allowing the sausages to sit after cooking so the inside can warm up. Experimentation will be the key here.