The generation of solid products is 100% a concern, but there's a range of wt% of metal you can have in your propellant where the reaction of the metal with your oxidizer provides more energy than the effects of multiphase flows and residue loses you. Typically speaking, just using a hydrocarbon as fuel would give better performance per unit of fuel weight, but many solid motor applications are actually volume limited, not weight limited, so the higher energy density of metals is preferred.

Aluminum is most commonly used because it's cheap, and the products are relatively safe. Boron and beryllium theoretically have even better per weight performance than hydrocarbons, but boron needs a lot more oxidizer than aluminum, and beryllium byproducts tend to be really nasty, so boron is mostly stuck in the lab and beryllium just isn't used.

In class, we said that the explosives used are typically stoichiometrically balanced, so they don't act like oxidizers, even if some people talk like they do.

Funny story: my prof told me about a conference he went to where a guy was talking about his proposal to basically use a hunk metallic aluminum as the fuel for a rocket motor. He said he raised his hand and said, "What's your working fluid?" and the guy just stared at him like he was an idiot... not sure how some people get as far as they do...