What happens when you "thrash" a steam locomotive -- operate it at the limits of its adhesion at near full forward gear for long periods of time. What accelerated wear can this impose? Do you get cinder cutting of the flues and blastpipe? How does the damage from hard use differ from that of a Diesel locomotive?
The actual damage tends to be worse when you 'link up' too quickly upon acceleration to "use expansion more efficiently" - this is when peak MEP enhances a tendency to slip but there is less back pressure to retard the slip when it starts, and there is more peak force and shock to bearings and rods. It turns out to be better to keep the cutoff fairly long at starting and 'drive on the throttle' until the train is reasonably well under way, even if you sacrifice some theoretical thermodynamic efficiency in the process.
Naturally much of the wear and damaging loading of a steam engine being worked close to its maximum TE (as opposed to dbhp, which I'll take up in a moment) is helped by things like Franklin self-adjusting wedges, driver and rod roller bearings, less deflection in the rods and axles, etc. The difficulty is that many forms of 'wear' are both cumulative and synergistic, and even a small amount of play or lost motion can rapidly propagate, and if you have a lubricated joint or, worse, a gland, there are further issues.
The thing about 'down in the corner' flogging is that you run out of steam rather quickly, and that limits some of the long-term despair. If you pressure-cycle the boiler (as well as thermal-cycle it with hevy firing to try to 'make steam' you can get into the kinds of problem that killed PRR 6200, but I think that's a bit different from what is meant by 'wear' here.
Yes, if there is heavy working leading to high draft, especially if firing is done improperly so there is lift and carryover, there can be excessive cinder cutting, not only in the front end but on the bends in the superheater header (some of those are armored against cutting in the first place).
Most of the 'derating' issue reflects an effective mis-specification of the stremgth of components in the running gear, rather than 'too much power'. In some cases you have a kind of complex effect where the running gear is overstressed and at the same time the MEP is higher than design spec or formula indicates; one way this can happen is when high-speed superheat becomes excessively or unpredictedly high (this has happened notably on Ross Rowland's 614 on a number of occasions). I believe a good example of the situation was the 79" drivered Niagara; another is the KCS 'light' 2-10-4. Yes, you can have the poblem at lower pressure, but normally the 'overbuilding' for the expected shocks in railroad service has already more than made up for any lightening. In the Niagara the lateral deflection of the lightweight rods was apparently underestimated, leading to a few distinct surprises...