I can only speculate that it has to do with the miners software on those things which doesn't seem to like the pool
For mine it was the 6-20 times the server died per day.
Right, and now there are no interruptions on the server side... none. nada.
I've been playing with Stratehm stratum-proxy before trying to mine on this pool with an S9. I'm not quite ready to deploy it, but I left it running overnight without any mining.
In the last 14 hours, there were 41 disconnect errors in the log. So unless there's some bug in the stratum-proxy, or tuning that needs to be done for this pool, it sure doesn't seem rock solid to me.
Of course, F2Pool isn't much better with 38 disconnects over the same time period.
Compare that to Kano and CK Solo pool, both of which had zero disconnects.