Now this is a subject I know all too well; having posted about it years ago. Unfortunately, they were removed from this forum update and put into the archive which I have no understanding how to access it.
AFAIR/AFAIK, PCMCIA is 8bit and Cardbus is 16bit, and the Pre-Intel Mac Books only use PCMCIA. Putting a Cardbus into the slot and it will not work at all. Somewhere I read the PCMCIA is slower than IDE due to the controller it uses (Mac or PC). But having a PCMCIA back up on a CF Card you can throw into the slot is great. If you can find it, you can use a PCMCIA FlashRAM Card (I seen them as big as 4GB by SanDisk), and it would be just as good.
The thing is the speed of the CF Card, a 100X is just slightly better than a Had Drive in its data transfer (if on the IDE Bus), a 200X CF Card is less than double, a 250X is about double, and so on (again, on the IDE Bus).
The 5300 is a clunker of a machine at times, but its the machine I love (my 5300ce). Compared to the 1400c I used to post the information in about using a CF on the IDE Bus, I did start using the CF on both machines on the PCMCIA Bus and it worked though it was slower than working on the hard drive, but it was bearable. transferring them to the IDE Bus was a major improvement.
I've encountered defects in the cheap eBay style CF-IDE 2.5" adapters, they are just a bridge so look out for something which has a master/slave jumper and looks OK.
In my experience (years ago) Single CF -> IDE Adapters work fine on the Macs. Dual CF -> IDE Adapters never worked on the Macs. From the 68K PB190s to the G4 PowerBooks and iBooks - they never worked. This is even though the circuit is clean and share all the connections but the only difference is the Master/Slave Switch on either side.
Somewhere online there is a kid who claimed to have had a Dual CF -> IDE work on a G3 iBook. Just in trying what he done, it never worked for me. But back then, I questioned it, you have a hard drive and a CD ROM Drive sharing the same IDE bus. With a Dual CF -> IDE Adapter with 2 CF Cards on it would never work for that reason - you can only have 2 devices on a shared IDE Bus and he had 3!
Now in my time, and the dozens of Single CF -> IDE Adapters I bought, I only had 1 bad one out of a lot. Even with no electronics on the card, a bad trace(s) can kill an adapter. I think that was the case in the bad adapter I have. No matter what CF I used or what machine I put it into (Mac or PC), it was dead. But out of the dozens I got only to have 1 dead board, I'm lucky.
BTW - on my work with old ThinkPads, the Dual CF -> IDE Adapter seem to work if they do not have a CD ROM Drive. Same with old Toshibas as well.