I've got a vader, and never an issue. I think it's only a cosmetic thing without woodgrain painted on. Granted, Atari always used what's on hand to make their consoles, so there's likely different boards in them not necessarily determined by it being a vader.
Right, and that's something I've been wondering about: is this is just a "rule of thumb" thing? That most versions of a certain model will be fine, but not necessarily ALL of them? It may be impossible to know with 100% certainty.
My 1981 four switch has a ton of interference with Pitfall II, Stay Frosty II and won't play Stella's stocking title screen, music resets over and over. It is unmodified. Searching threads this has been reported before but not nailed down to any revision or reason.
Maybe there is no 100% compatible 2600 then. It was seeming like the four-switch woody was the closest, but after hearing this, who knows. I can sort of understand the homebrews, but Pitfall II acting wonky is particularly troubling to me.
To echo what basically everyone else has already said, the only "incompatibilities" I've encountered are:
Six-switch systems (heavy or light doesn't matter; they have the same cartridge interface): Certain third-party cartridges and "fat" Atari Corp. cartridges fit very tightly, but they run.
7800*: In rare instances, 2600 games basically work but exhibit some incorrect behavior that can cripple their playability (ex: Mario gets stuck on ceiling border in Princess Rescue, making the game impassable at a certain point); Tigervision and possibly a few other 3rd party 2600 cartridges don't fit.
*The thing about 7800s is that they're kinda all over the map, with numerous minor hardware tweaks and revisions that can affect compatibility. Not all Atari 7800s are created equal. FWIW, I have two 7800s--one earlier one with the expansion port and a later one without; the one with the expansion port is the one that has some weirdness with a few games (even some 7800 ones!), while the other one hasn't presented me with any compatibility problems (so far).
Yep, as I said before, the only real issue I ever had was getting Atari Corp. shells into a light six-switcher; they played, but it was tough getting them loaded and unloaded. Don't think I damaged any labels, but there may have been some scuffs on the back of the shells, don't remember for sure.
As for the 7800, it seems like there were certain revisions to look for, but I don't know, it's kinda like you can just never tell with that console.
Can anyone explain the heavy sixers having problems with some games? I saw a comment about it while doing a search recently, I thought it mentioned that it was with certain games that used bank switching. Is there a list of games that prove problematic with heavy sixers? (And, I'd guess there's not a whole lot of variation between those models, since they were only around for about a year, but are there differing revisions? Maybe between the Sunnyvale models and the less-common ones manufactured in Taiwan?)