What are some alternatives?
When comparing mergerfs and Greyhole, you can also consider the following products
StableBit DrivePool - State of the art, file based drive pooling software.
mhddfs - The driver combines a several mount points into the single one.
Drive Bender - Drive Bender, class leading storage pooling technology
unRAID Server - A scalable, consumer-oriented, server operating system.
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
FlexRaid T-Raid - Unify & organize your data. Protect & recover from disk failures!