I've swim in a few man made lakes, and a few natural lakes in alps, Scotland etc. I don't think there's much difference except that reservoirs are more likely to have signs saying no swimming. having said that, several of the big reservoir lakes in Wales, they let swimming happen, and there are also a few in England.
obviously don't swim right near water intakes, or jump in before checking the depth, or go in too quick when the water is cold, same as any other outdoor water you might swim in. be aware of hypothermia, wind, and be realistic about how.far you can swim if you're heading across a lake.
if there really were strong dangerous currents (in flat water lakes?) they wouldn't let people boat on them, and they'd be a whole lot more exciting to boat on.
Wherever you swim outdoors, people will tell you it is too cold, too strong currents, too wide, too deep, too shallow, too narrow or generally too dangerous in some way or other. mostly, people saying this are idiots with no knowledge of swimming, or the water concerned, you can spot these cos they'll warn you about dangerous currents in lakes, or about the water in rivers in summer never getting above 5 degrees c and other bollocks. if you're not sure about a piece of water, find someone who actually swims and ask them.
Joe