My apologies for the delay. Gorgonops is spot on here - I'm exaggerating a little for dramatic effect.
The MDDs run OS 9 fine. They're not known to be unstable relative to older machines. The only real "problems" with OS 9 on MDDs boil down to these points:
- Mac OS 9 handles MDD fan control poorly and may cause the system to run louder than it would under OS X.
- Installing Mac OS 9 the officially-sanctioned correct way on an MDD is inconvenient (but this is true of several earlier Macs as well, including the QS'02 and the TiBook/867-1000.)
- MDDs may have graphics cards that do not have OS 9 drivers installed (but this is also a potential problem with earlier Macs).
- When buying an MDD you may not be able to receive a guarantee it is one that officially boots OS 9.
- because part-swapping over the last 20 years can result in machines that have non-original bits
- because some sellers do not fully document systems
- because some configurations/items e.g. dual 1.25GHz were available in all three MDD sub-generations
The arguably more important point is the one Gorgonops has captured perfectly: MDDs are nearly all entirely overpowered for OS 9 and even when one is configured within 9's limits it makes poor use of the hardware relative to OS X. (This is also true of many earlier systems. OS 9 makes poor use of all dual-CPU Macs and it makes poor use of networking and storage hardware on all hardware it runs on.)
My experience (and if I had to guess this applies to all but a dozen or so people who touch Mac OS 9 on a regular basis) is that Mac OS 9 and most software performs very very similarly on basically everything from a G3/300 to a G4/1000. I have 9 loaded on PowerMacs G3/233, /300 (beige) /450 (blue), iMac/400, iBook/366, QS'02/800 and TiBook/1000, along with a QEMU-PPC instance running on an intel i5 PC. In my 9-era day-to-day it's nearly indistinguishable on all of them.
In my experience, Mac OS 9 isn't stable enough to run more than a handful of programs at a time and almost no single program requires more than maybe 30-40 megabytes of memory (netscape 7/classilla are probably the most ram-hungry applications on most people's Classic Macs, if i had to guess.)
I have seen people *(not very many, but when I asked, they doubled down) claim they can perceive the difference between highly upgraded QuickSilver 2002 and highly upgraded Mirrored Drive Door Macs when all other configurable stats (cpu/graphics/disk/ram) are equal, but to be perfectly honest I don't believe them. I think you can measure or benchmark the difference, but not feel it. I can't even feel the difference between a G3/233 and a G4/1000. Having an "ultimate" OS 9 machine might matter if you were using software that was doing {render|process|compile|whatever} work very very regularly.
But - most of that software runs and works meaningfully better under Mac OS X anyway. OS X makes better use of literally every component of a Mac, for all work -- for everything from writing in textedit all the way up to rendering and compiling simultaneously while still reading news and composing email, or whatever.
My genuine recommendation if you want an MDD PowerMac G4 is to consider running OS X on it. Exclusively.
These are super neat machines and after a decade or more of being actively disinterested in the idea of "retro" or "vintage" Mac OS X, I, myself, am coming around to the idea that I might have a renewed interest in a couple old versions of Mac OS X. I've already got a few solidly X-era machines, but I have explicit plans to add an MDD.
I think it's cool that some of the MDDs run OS 9, XPress Edition has been one of my favorite jokes about the Mac platform and Quark for eighteen solid years, but I also personally think that the MDD is better enjoyed as what it was really meant to be: a powerful G4 OS X machine. For OS 9, "last" doesn't necessarily mean ultimate, and when considering the wider context of Macs, "ultimate" for OS 9 is really where OS X steps in and shows its strengths.
Hopefully this is helpful! I'm just some guy on the Internet and if you still want to run OS 9 on an MDD, you definitely should not let me stop you. It just... might not be much better than your blue-white G3.