That's a pretty good description of a canned tune right there.
Let's say I take two TOTALLY bone stock 93 Cobras with exactly 5000 miles on each. I can tune ONE of those two cars and apply that tune to the second car without too much trouble. THIS... is a canned tune.
Now... let's put 50,000 miles on both cars. Car A lives out in the North East. Car B lives in Arizona. Each one wears differently, sees substantially different driving conditions, but both car owners are pretty good at keeping up with maintenance. Car B blows a head gasket one day and overheats. It gets a brand new set of head gaskets and also gets the heats cleaned up and resealed along with some minor grinding on the head surface to make them flat again for installation. The headers are OEM and have seen better days, so they're tossed for some 1 5/8s shorties. Aww hell... might as well put some free flowing cats on there too and oh... The Bassani cat-back REALLY sounds nice. You get the idea here....
Even if car B had NO modifications, it's going to wear differently than car A. With modification, that wear is going to be even more severe. That doesn't even take into account whether replacement items were done correctly on either car. Every Fox on the planet takes in some amount of unmetered air. No two engines are the same. One gets a little more air than the other, one has an injector that's slightly better/worse, thorttle bodies, ignition, PCV, etc...
Now you can build a margin of safety into a canned tune but the more you modify and the longer the engine runs, the further you're going to be from the center of that margin.
THIS is where the problems start to come in. Performance problems and also problems sometimes so severe, that it causes engine damage. Hell... I've been in car that ran and FELT great! Until that day where conditions pushed it just past the point that made the whole thing go, "POP!". Lean engines make power (to a point). That 93 octane tune you got from Tuner X? That's what's been done. The fuel mixture has been leaned out, and timing advanced to give you that extra XX HP. Unless you've got some way to check though (the new GTs are great... they have a built in AFR meter), you're taking a risk with your gear.
Sure... some of these tunes are better than others but the point is that you take it on blind faith. After having been part of this discussion (as others have been), if you end up one of the dudes on the classifieds here who's parting out his roller well....
NOT EVERYONE WHO MODS THEIR MOTOR NEEDS TO KNWO HOW TO TUNE... Everyone who mods there motor DOES NEED to be able to understand the basic principles of tuning what it does to/for/against your car.
If you're not sure what an injector slope or pulse-width is... then you should probably find out for the SOLE purpose of understanding what can happen when they are wrong.