It isn't the NTSC machine, as the NTSC machines can display PAL. It's the screen you're outputting to and whether it can display PAL or not. PAL games on NTSC systems run in PAL. Trying to get them to run in NTSC will cause problems.
If you don't believe it, set a NTSC tooltype in any number of PAL WHDLoad games (try for example super stardust). After a few seconds of loading, it will switch the monitor to PAL mode even though you tried forcing it to NTSC.
Other games will simply refuse to run (many) or have glitches (rtype 2 slows down to about 10fps, etc.)
I'm looking for NTSC versions of so many games it's ridiculous. At the top of my list are Super Stardust (and cd32 version), alienbreed tower assault, disposable hero, banshee, etc.