The only reason for not using this superb add-on would be not knowing about it. OK, now you know!
And the only reason to criticize it is most likely lacking elementary computer skills. If you read this short review and trust it, you probably not among those.

It easy to use once you get familiar with its UI, has few issues and gets updated regularly as far as I noticed. It supports many sites, way more than described. Be advised, for best results, especially when you need to convert video/audio formats, use a support application, referred to as "Companion App", which can be downloaded separately.

This being said, it is by far one of the best add-ons I ever used, both on Firefox and on Chrome (or derived from it). I like it a lot and would recommend it as a must for all those willing to enjoy the streamed content off-line.