Is the 2010 Haro Railer XS good as a road bike?

Strictly speaking it's a cruiser, not a road bike. Road bikes traditionally have drop bars and a more hunched over riding position.

As a cruiser it has an upright riding position, making for poor aero characteristics and difficulties riding standing up out of the saddle.

It's OK for casual, short-distance riding but not the bike you want to use if you're going far.

Having an alloy frame and a 3-speed hub makes it a little better/lighter than many other cruisers, which isn't saying much. You'd probably want to get another sprocket to get the best use out of the available gearing range.

Also, what'd make the bike more useful for everyday riding is fenders, and a way to carry stuff. A rack or a basket.

The stock rear fender is a joke. At that length it won't even protect the rider from picking up a "racing stripe" if the road is wet, which is kinda the minimum requirement for a fender to make any sense.

And if you like to ride in company, you'd want a fender with a lot more coverage, or you'd have to stay at the rear of the pack on wet roads to keep your pals out of your rooster tail.