Good question, I don't really know. If I had to guess I'd say you're right: the internal thread chaser is less aggressive (
). I suppose if the threads are
really damaged, the chaser would only focus on the damaged areas, whereas the tap might wander when it encounters these. That could either make it break off, or cut into the existing threads and damage them. I guess this falls under the aggressive description you mention.
You know that feeling you get with a tap when it is cutting just right and you've figured out how much pressure is needed to cut well and when to reverse? I never get that when using a tap to chase threads: it's just a nasty bump against the damaged area and a prayer that it won't break as you push through. I'll bet the forces on the tap are concentrated on one area rather than spread out across the tap, hence the likelihood of it breaking.
I'm not adverse to internal thread chasers, but the NES set was pricey if I recall. I needed it mostly for externals and so compromised by not getting the more expensive set with internal chasers. A lot of the internal ones I have fixed over the years by plug welding and then re-tapping. If I were doing internals more often then I'd consider buying an internal set.
BUT if I do break a tap in there I want to have this guy help me: