Unfortunately, appliances are like other types of 'tools'. One manufacturer makes the best dishwasher/saw/combine. Another makes the best refrigerator/drill/planter. Someone else has the best oven/planer/disk. Not a problem in the toolbox or in the shed, but mix/match can be a problem in a kitchen because 'stainless' or 'white' aren't necessarily the same across brands, and you want things to look coordinated.

What to do? Maybe start with the biggest and most visible- the refrigerator. Look for one you like and then think about other items from that manufacturer. Are they acceptable? Are they near enough in your kitchen layout to need to look good together or are they in different site lines? For instance, a cooktop is not even in the same plane as a refrigerator front, so should be no problem using a different brand.

Certainly cost, availability, and dealer support are important as well. A dealer may offer some discount for buying all one brand (theirs), but is the savings really worth settling for a machine that's not what you want or is known to be a poor performer? Distribute that 'discount' over the expected life of the appliance, factor in a few headaches, and the deals start to look less and less attractive.