Why does Apple not recommend storing your iMovie library on an external drive?

I want to try keeping my iMovie library on an external drive because I use both my iMac (at home) and my MacBook Pro (when traveling) to edit my videos. Ideally, I would like to use a SD card so that it doesn't draw power from my MacBook Pro, but it may be too slow? It will also free up hard drive space on both computers. But when searching about this, Apple's support page says it's possible but advises against it: > "You can store your library on an external storage device such as a USB drive, FireWire drive, or Thunderbolt drive formatted as MacOS Extended (Journaled). Apple doesn’t recommend storing iMovie libraries on external storage devices like SD cards and USB flash drives, or drives shared on a network." Does anyone know why this is so? I want to make sure it's not damaging in anyway.