Typical SSDs are rated to 1million write cycles. Cheapie CF cards are rated to 1/10th that. However, many of the faster CF cards (266x and 300x+) which you'd really want to use for a hard disk replacement anyway, are rated to the same 1million write cycles as your typical SSD. SanDisk sells industrial CF cards rated to twice that. Additionally, unless you're really using all the space, most of the unused flash blocks will still be used to distribute the write cycles.
Low end CF cards also have a bit of a stutter to them as the wear leveling kicks in which is largely addressed by the higher end cards that also typically have the higher write cycle rating, which is why I say you'd really want to use the higher end CF cards as a hard disk replacement regardless of write cycle concerns. And even at the higher end CF cards, it's a more cost effective solution IMO.
None of that addresses the form factor and ease of mounting concerns though. The SCSI-IDE converts and IDE-CF adapters are a mixed bag in that department. They do allow for some interesting mounting options, such as the CF card accessible via an unused floppy bay or nubus slot. But typically require much more time to mount properly regardless of the CF accessibility mounting or stock internal drive replacement mounts.