I can see anything with Triacs not working with DC, and would be curious how common Triac motor drivers are as opposed to ones using transistors or mosfets. Are they used for the open frame motors, even though those could be run from DC?
As for TMCC engines, the track signal requires AC as a carrier, so anything TMCC or Legacy technically needs to be powered from AC. I think one could get around that if they really wanted to, but that is a whole other subject. Aside from a triac motor driver, however, I do not know what could be actually damaged in TMCC or Legacy from applying DC power.
As others have mentioned, anything that depends on the DC offset for sound control will be a problem, even if the engine its self would run on DC.
When it comes down to it, pretty much all O gauge trains can be run on DC, if you want to make some modifications, and the same is true of running "DC only" ones on AC. The main limiting factor here is if you want to keep all the original electronics working correctly.
For those more knowledgeable on these things, without modification do typical post war engines run on DC? I'm not sure why they would not, but I don't know if maybe the e-unit has to be wired differently or some such. I know there is some talk of overheating the e-units on DC power, but not familiar with what causes that either.
From time to time I've kicked around the idea of using DC power, mostly due to high current DC switching power supplies being so much cheaper than AC transformers, but in the end the cost savings hasn't really worked out with the costs of getting all the trains working correctly. I still plan on using pretty much free computer power supplies to power lighting and accessories.
JGL