This seems like it has made progress since I last looked. My personal experience. I had TMCC first, and yes it was easy to hook up. When I added DCS, I used the DCS handheld for MTH engines and the cab1 for TMCC engines. Two remotes two systems but it worked pretty easily. At some point, and I don't exactly remember when, I added some of my TMCC engines to the DCS handheld. I didn't have any issues, and was doing basic operation. When the DCS wi-fi came out, I really enjoyed it and stopped using the DCS handheld altogether. I prefer using the iPad to control the trains. Adding a Legacy locomotive will work for basic operations on the DCS wi-fi app, but for some features it won't, and I get that.
OP, it sounds like you may wish to do a factory reset on the engines that are not responding. You also may wish to check to see if you have the lights on and the volume up. If you can get the engines to work using a Lionel handheld, you should be able to get that to replicate to the DCS handheld.
I think the comparison Mac to PC is spot on.
John