While a modified sine wave inverter will get the job done in almost every circumstance, there are some cases where it may cause damage or just not be very efficient. The primary category of devices that run more efficiently with a pure sine wave inverter are electronics that use AC motors, like refrigerators, compressors, and microwave ovens.
They’ll still work in most cases, but they may not work as efficiently, which may lead to excess heat buildup and the potential for associated damages.
If you use a CPAP machine, especially one that includes a heated humidifier, then you’ll probably want to go with a pure sine wave inverter to avoid damaging the unit. It’s always a good idea to check the recommendations of the manufacturer, but most CPAP manufacturers recommend going with a pure sine wave inverter.