They wipe out large areas of spectrum. The UK Amateur Radio Licence used to have a sentence explicitly forbidding the use of spark transmitters, but it no longer does. The regulatory authorities perhaps assume that no-one would be silly enough to try. The spark transmitter is very simple, but it generated a large number of technical problems mostly due to very large induced e.m.f.'s when the spark struck, which caused breakdown of the insulation in the primary transformer. To overcome this the construction of even low-power sets was pretty hefty.
Low power is a relative term.
The wavelengths used were from about 50 to 6000 metres. The circuit diagram is shown below. When the key is closed, the condenser C 1 is charged nearly to the voltage from the secondary winding of the transformer. A unit of capacitance widely used until relatively recent years was the 'jar'. The inductance L 1 together with the spark gap and C 1 produces the high-frequency oscillations, which are transferred via L 2 to the 'open' circuit.