Hey, just found your thread on Google as I wanted to access FoxMixer. Just want to share my 2ct's on this topic.
I've been using FoxMixer for over 1 year now, also for pretty large amounts (imho).
There has been no issues at all. The reason I prefer to use FoxMixer is the mentioned fact that they're not that hugely advertising their service. I've realized a lot of well-advertised services getting busted due to the fact that authorities have a high interest in getting the data from them. And they've outlasted a lot of other "trendy" mixers. If you look at Chipmixer as an example, they recently had a huge DDoS attack (several times already).
Plus I always prefer to have multiple options.
By the "last-move" dates of the coins that you receive from FoxMixer you can also tell they're used quite a lot. And I've not noticed a lot of complaints after all on their thread, nor did I notice any downtime in this one year.
Btw, they're also part of the open source double-mixer project which I find quite decent, see https://github.com/teran-mckinney/doublemixer-python
If you've further questions just ask here or send me a PM.