Line level/instrument level and balance/unbalanced are independent attributes. That is, either type of cable can accommodate either strength of signal. Pro sound usually uses line level signals and balanced cables. Guitar players usually use instrument level signals and unbalanced cables, but the attributes aren't technically tied together. Signal strength is an attribute of the output of the device, not of the cable.
Balanced cables are three-conductor cables. For cables with quarter-inch plugs, you can tell by whether the plug has two conductors (tip and sleeve = unbalanced) or three (tip, ring, sleeve = balanced).
There are no safety issues here. You can use either type of cable with either strength of output with no danger whatsoever.
The advantage of the balanced output is that you can run your line over long distances when you use balanced cables. With unbalanced outputs, you want to limit yourself to fairly short runs (maybe 20 feet) before capacitance starts to affect the signal quality.
If you use unbalanced cables with a balanced output, you lose the advantage of having a balanced output.
If you use balanced cables with an unbalanced output, there's no advantage nor disadvantage, except maybe you spent too much on your cable.