Is there any concerns (food safety, quality, otherwise etc) with storing dry goods [spices, protein powder], or bottles [wine bottles, pop cans] on top of a refrigerator unit?
The top of your fridge might tend to be a relatively warm spot in your kitchen: it's up high, and the fridge itself gives off some heat. Exactly how warm would depend on your kitchen and your fridge.
So it might not be the best spot for spices, since heat can make them age and lose flavor more quickly. Some other foods also prefer relatively cool storage.
Otherwise it should be totally fine; most things can handle being slightly warmer than the rest of the kitchen.
No. There are no issues with using the top of your fridge as you would a pantry.
I do it all the time. Wine bottles (for short storage, usually until the weekend ;), Bags of chips, some bread, etc.
Your fridge doesn't output any fumes or chemicals (it's a closed-loop system). Other than light getting to your items (which may shorten the "shelf-life for some items), there's no risk or hazard.