People have had successful tanks with canister filters but they are not very well suited to SW
in my opinion. It is going to depend a lot on what kind of tank you want to set up. SW
tanks with coral are going to need a lot of flow. It is recommended to have 10x turn over rate or better. It can be difficult to provide that with a canister on anything except a small tank. A good canister provides three types of filtration. (btw i'm just starting with basics i do not mean to insult if you are a more advanced hobbiest.) First is mechanical, which is usually in the form of a sponge that catches particles. In salt and fresh this will collect a lot of garabge and can easily contribute to high nitrate levels if not religiously maintained. Second is your chemical, this is where a canister really does have some application. It is an easy place to run which ever kind of media you wish (phosphate, carbon, or others in unusual cercomstances). Last is your biological. There really is no need for additional biological filtration in a SW
tank because your live rock and sand bed will provide more then enough surface area for the benifical bacteria to grow. So when it comes down to it, to me anyway, a canister filter is really just a pump with the option of running media conviently.
I would rather see a sump/refugum combination. It will provide space for equipment, nutrient export via the macro, and additional water volume. Running media is not difficult to accomplish either. As far as power outages, this is an easy problem to fix. When you set it up, you unplug everything and then top off the sump to just below the rim. Then plug everything back in and mark the water level on the tank. This is your max water level for the sump and then if the power ever goes out you are positive that you have enough room in the sump to accomidate the additional water.
This is just a ruff outline though. Depending on the type of set up you want this can be tailored to best suit your needs. I guess the shorter answer would be that in my opinion i don't think canisters have much application in SW