You're wasting about 2-3% of the transformer rating in inefficiency. If you're switching secondary voltage, then you're paying for the primary winding losses 24 hours a day even if there's no load on the transformer. The transformer is just one more item that can fail and leave you without your appliances.
The bigger problem is the transformer is going to set you back about $100, so it spectacularly fails to make sense for a $40 humidifier.
Dollar for dollar, if you're coming to North America, you may be better off just adding a couple 240V circuits to your house, and running the appliances off 240V straight shot. North America has both 120V and 240V ...
Something like a dehumidifier which has a fairly high load and a high start up current, will require a larger transformer. Cost and portability will be major considerations.
Smaller appliances with low current draws, will be easier to fit to a small transformer.
For reasonable quality transformers, figure a loss of about 3%.