FoxDen tended to work slightly better for very small video conferences. The audio was typically reliable, so it would be ok to use for audio conferencing. It is definitely not ready for prime time: it cannot handle very large conferences. It didn't seem ready to be used on mobile either. I also think it's better suited for a smaller company.
FreeConference.com is best suited when the big-name video conferencing brands are simply not working. It offers an alternative when compatibility seems to be an issue when attempting to video conference. It is less appropriate for first invitations since most people are not familiar with the brand.
Horrible video quality. It basically never worked, and we stopped using the video feature.
Buggy behavior all the time. Dropped calls, loss of screen sharing, rendered unusable most of the time.
Poor response by the vendor to issues. The answer was basically "we know it's unusable, and we're trying to figure that out. There's nothing we can do right now."
I believe they selected FoxDen because we use ReadyTalk for voice conferencing and FoxDen represented a cost savings compared to our previous Skype for Business investment. However, in the end, most of our company has abandoned ever using FoxDen and, instead, still uses all of the products listed above (Skype for Business, Google Hangouts, Slack) or just plain ReadyTalk voice conferencing.
Call quality is very similar to the above products. However, most of our community partners and collaborators are not aware of FreeConference and are thus less able to utilize it without prompting and sometimes assistance from us.