I disagree. I know more people with CIB Turbo Grafx collections including outer boxes and all the Super CD releases than CIB NES and SNES collections. There are 7 times as many games for each of those systems, and some games like Stadium Events just don't pop up often with a box ever. I mean less than 5 times I have ever seen a boxed Stadium Events on ebay for NES, and that's pushing it.
I would say Game Boy, Game Gear, Colecovision, and Atari 2600 are the roughest to collect CIB games for. Even if you have money, some of those still never show up.
If you're judging "finding" and "collecting" as 'collection completing', then sure anything with a large catalogue would be harder. Either way, collection-completing-wise, all it takes is a single impossible-to-find-complete title to make it technically "the hardest".
But if we're talking "Hardest system to find complete games for", then any console with 7 times as many games for it will be
way easier to find complete games for. There are less than 150 TG-16 HuCard and CD games, but how hard do you think it would be to find 150+ complete NES or Genesis games?
At least half of the TG-16 HuCards released can be easily found for sale in new/sealed condition. There seem to be very few incomplete jewel-case-only TG-16 CD games in the world and most of the CD games that came with boxes aren't hard to find at least complete, with many regularly around that as new/sealed.