As many know and are probably tired of hearing about; I’m an infamous bee hive heater :-D
My experience would concur with the advantages Mushmushi listed.
As for the disadvantages, here’s what I found:
Concern: Difficult to control the heating levels and worry about cooking the bees.
Response: This depends upon what your goals are. If you only want to give the bees a break from a forecast week of an arctic blast (ie -20C to -30C spell) you could simply plug in a 10Watt heater (or incandescent light bulb) and let it run day and night through an extreme cold spell and never have to worry about cooking the bees when it is so cold outside. 10watts doesn’t warm a hive by much when it’s -20C outside! However the warmer convection currents rising up around the bees cluster may be greatly appreciated. If your goal is to minimize honey/stores consumption in the winter, then you would need a little controller to maintain the hive temp around 5 or 6C (low 40sF). If you’re trying to baby a small colony through winter, then 15C (60F) works well.
Response: Yes, there is cost with electric heating, but there is also cost with buying new packages every spring. All my electrical parts cost less in total than a single new package of bees. I use an old computer power supply for power and some homemade cement/resistor things for heat. Total cost for a simple system is the cost of a bag cement, about $1 of resistors, and an extension cord. Plumbing heater cables have also been used by some for heaters. Cabling is the most expensive part. If you want to control a hive to a fixed temperature then you need a thermostat for the heaters. For DC heaters you can make one of those out of a simple op amp circuit (no programming needed) for under $5 in parts. If you want a more sophisticated controller and know how to program a micro controller, you can select from a ton of low cost 8-bit micros that are more than capable of reading temps from a thermistor and controlling the heaters with FETs. An 8 bit micro costs $1 to $4 depending upon what you want.
Concern: No overwintering gains?
Response: You lost me here, I don’t know what you mean.
Concern: Dryness in the hive.
Response: Yes, this can be a problem in the spring. Heated hives are very dry. You never have to worry about wet bees in a heated hive. Come brood season, I think you would want to add water to your hives somehow. A sponge, a coffee/food lid filled with water, or water poured into comb.
Concern: Electromagnetic Radiation
Response: EM radiation is generated by large current flows (your power company lines) and high frequency noise where there is a conductor that acts like an antenna and impedances are not matched. You really have to TRY to build a transmitter if you want to make a significant level of EM radiation. There will be more EM from cell phones and power transformers than bee heating equipment. If you go with a simple Christmas light heater, how much EM is in that? My stuff is powered by a computer power supply. Those use switching in the low MHz range to generate DC voltage. That does generate some noise, but you need a pretty long wire (antenna) to radiate 1MHz.
Concern: More work for the bee keeper.
Couple of items you didn’t list:
Concern: A warm hive will be more conducive to pests like wax moths.
Response: Yes. Unless you’re babying a nuc, it may be unwise to heat the hive above 12C (55F).
Concern: What if the power goes out? Do my bees die?
Response: If the power goes out, your bees will have to cope with the cold like all the other bees out there. An extended period of cold may do them in, just like all the un heated bees. However a power outage does not mean immediate death to your bees!
Concern: The bees will keep brooding all winter long.
Response: I have not seen this with my carniolans or Italians, but I never say never.
Concern: Electrically heated hives will run my electric bill through the roof.
Response: No it won’t. Running a 10watt bee heater will be under 5 cents per day in most of the USA. So $1.50 per month. Most of us don’t have -20C all winter long, maybe you just run a heater for a month or put it on a timer? The power cost depends upon how many watts of heat you want to pump into your hives and for how many hours. Say you add a 10 watt heater to your hive and run it 24 hours a day during a cold spell. That is 10w x 24hour = 240watt-hours of power you need to buy from the power company per day. Electric power is priced by the kilowatt hour used in the USA. In the Midwest power usually costs about 12 cents per kw-hour (can be double that on the east coast, or Calf). So 0.240kw-hours x 12 cents/kw-hour = 2.88 cents per day to run a 10watt bee heater. How much does it cost for new packages of bees from the south? How much is a pound of sugar? Electricity is cheaper.