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.
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.