Factory brine vs home brine
I think (as is so often the case with preprocessed meat) it will often depend on who you buy it from. I often buy kosher prepared birds (which are always pre-brined salted) and I've never had any complaints. The nice thing about kosher preparation, is that it is done according to very specific rules, under strict supervision. It's not a factory process.

On the other hand, I cannot abide the brined birds from major poultry companies. Their primary goal is to increase the weight of the bird, not the flavor.

Factory prep seeks to minimize time and cost. They will use cheap ingredients (largely just water and salt, with no added herbs) and they will use whatever process takes the least time, most likely injection, which has the added benefit that they can force more liquid (and therefore weight) into the meat than would normally come from absorption. They'll likely insta-freeze it, so wait time is probably not an issue, but freezing causes its own issues.

I would say, for best results, take your time. A nice leisurely soak, real salt, and no pressure on the meat. For best results (talking turkey here) look for "Natural" or "Minimally Processed": if it says that, you shouldn't have to worry about competing additives.