The worst site for disconnections has been poker4ever. I know this is a new site but I have had more problems there than all other sites combined. The last time it happened they actually paid everyone still playing in the tournament.
Just recently I had a problem at
Titan. In fact I think it happened during the BBP freeroll. It was a little frustrating.
Poker.com had some problems a few months ago. When they upgraded in December I think they were able to resolve those issues. Recently they had a system upgrade and paid out all tourneys.
There are definitely some risks you take playing online. Not only do I have to worry about my own connection, there also can be problems at the site as well.