As long as the maximum current load does not exceed the current rating of any of the cables that are used in the extension, it will be safe. There should not be very much loss over a few hundred feet of extension. If there is a loss, you can then change the extensions for ones that are of a larger gauge.
I have run extensions of up to 400 feet. These were 10 Amp rated. I had no performance loss in the loads that I was using. The load was about 4 to 5 amps. I am working with 120 VAC at my location. I took a voltage reading, at the outlet, and one across the load. There was a drop of about one or two volts. But, the total voltage at the extension output was still in specs while under the load. The source voltage at the outlet was about 118 VAC. Across the load, if I remember correctly, it was about approximately 116 VAC.
In areas that are using 220 VAC, the current is about 1/2 for the same wattage, as when compared to 120 VAC. This would lead to less loss when using an extension cord.
Most devices here in North America are rated at 105 to 125 VAC. Some are rated from about 105 to 135 VAC.