We're considering a speed oven as a 2nd oven, probably 27". Our main range is an all gas 30" range. What are those microwave / traditional ovens combo good at? What are the best uses? Conversely, what should they not be used for?
Microwaves are fine for veggies and for reheating most leftovers.
They are notoriously bad at anything having to do with bread, or anything crunchy. Nothing will make a nice crispy coating go soggy faster than a microwave.
Some people like them for rice, I do not. I think they ruin rice.
You can quickly achieve an edible potato, but it won't have that fluffy interior or crispy skin of baked. By cooking a potato until almost done in the microwave, and then finishing it in the oven, you can get (most of) the best of both worlds. It won't be quite as good as a fully baked potato, but it'll be pretty close. I would think a combo oven would be a good choice there.
We have had quite a few questions in the past in which combo microwaves showed symptoms of not turning off the microwave part when being put in what is supposed to be a pure toasting mode. If this is true, the only thing such an oven is good for is the same thing as a microwave:
- liquids (warming or cooking them),
- vegetables, steamed-like,
- heating food which will be eaten immediately.
Personally, I wouldn't take the risk, and would rather purchase a standard toaster oven. A good quality toaster oven can do everything a large oven can do. I have been using a toaster oven exclusively since 2006, with a short break (1 year living in a place which had both a large oven and my toaster oven). I even got decent macarons out of it with a few hacks; things like bread and cake have never been a problem.
It needs a bit of getting used to (it bakes with more radiation and less convection at the same temperature, so the outer parts get crispy quicker). And it is not as even as a large oven can be. But if you pay attention to what you buy, it makes a great energy-saving second oven. For buying advice, see this question and read reviews. This is also a product category where reading test results by independent organisations can be very helpful!