What can be the problems or danger of running a motor slower by simply lowering the dc voltage supplied to it?
If it's heavily loaded, it may bog down and stall, whereas running at the original higher voltage, it would run faster and ride through a heavy or uneven load.

When stalled, it will draw a high current and dissipate all that power as heat, not motion, and possibly burn out.

If it's lightly loaded, so you don't allow it to slow or stall under load, it should be fine.