DOS was rock solid because it did nothing. Many programs, particularly games and anything that did networking, didn't even use DOS interfaces—they bypassed them entirely and worked with either the BIOS or hardware directly. There was no memory protection, no multitasking, and, on a higher level, no permissions nor sandboxing. So while maybe it "just worked", I wouldn't call it robust.
Modern OSes can run arbitrary binaries, but they can pretty much run arbitrary non-adversarial binaries - problematic binaries have to be intentionally written to exploit the system (as opposed to DOS, where non-problematic binaries had to be intentionally written to not break the system).
It's a dramatic improvement.
Mostly. Modern OSes strive to run abdersarial binaries, but where they can do it safely or not is still in question, IMO.
Since 2008, it's been a dramatic departure.