Are universal power supplies for laptops worth it?

The usual take on this is: They may work but if anything goes wrong, then don't expect your warranty on the device to hold. At least that's what the manufacturer will tell you.

The power just specifies how much the power supply is able to supply but it won't affect how much your laptop is going to use to charge the battery.

As for the voltage, using 18 or 19 V should work without damaging anything, but be sure to try with 18 first :-)

Bigger wattage is fine and will not harm your laptop. It just means that the power supply is capable of handling more load than your laptop will generate.

As for the voltage, 0.5V is such a small variation that it should not matter at all; the power grid will have such differences in the first place. However, it is better to provide too little than too much voltage, so stay with the 18V.