A well-known characteristic of CompactFlash cards is that they do such things as write-leveling and caching to boost performance and longevity.
These characteristics make them fairly poor choices as an SSD replacement (using a SATA adapter, or on an embedded board that takes them directly) because even if they report to the OS that a write has completed, they may still be busy.
Nevertheless, some embedded manufacturers still use CF cards as the means for primary storage. In particular, the Alix series of boards do this.
If write-caching is disabled using hdparm, will a CF card still do its own caching and such, or will I have guaranteed writes? Is there any way to guarantee a write has completed? Write-mounted journaled file systems rely on an underlying guarantee of writes completed when they say so, and so suffer routine corruption on CF devices in my experience in environments where the machine is power-cycled routinely.