Merge network is a collection of online sites that share a player pool. They are US friendy, and in terms of players/games probably have the biggest selection of any site that allows US players. As far as getting shut down; who knows. Your money is as safe there as it will be anywhere for a US player right now. To my knowledge there have not been any recent closings of sites that allow US players, but obviously it has happened in the past.
Most importantly, there is NOTHING illegal about online poker, unless you happen to live in one of a select few states that have passed state laws against it, but on the federal level it is not illegal, and never has been. They (Department of Justice) targeted sites and/or banks for using round-about methods to get the money into the players hands.
That's the part that is quasi-illegal; banks handling transactions involving gambling proceeds. But, from a player perspective, there is nothing illegal about online poker, unless you live in the few states I was talking about earlier. I'm not sure which states are on that list, but it should be pretty easy to find out with a google search. I think New York, Oregon, maybe Kentucky, maybe others, but you should check for yourself about your own state. Maybe someone on here will have a complete list, or a link to a complete list. I don't.
By-the-way, some of the biggest and most popular Merge sites consist of Carbon, Hero, Black Chip Poker, but there are many, many others; probably around 30 or so altogether.