Heavy they are, and slow to heat up, but many prefer them.
We use a fan heater with a high low setting to allow for sites with wonky power and it is very fast to provide some warmth - plus, provided it is not too noisy, is a good outside-noise-masker in noisy campsites.
Compared to oil heaters, fan heaters are a fraction of the weight and also usually roughly cubic in shape making it easier to tuck them away when not in use.
Also - if it is not stored correctly before driving off, I would much rather get belted over the back of the head with a fan heater than an oil-filled one.
For low heat when stored in freezing weather, an oil filled unit would be more reliable.