They're all solid, until they're not. Back during the housing crash, a lot of new companies popped up, trying to cash in. Now that foreclosures have returned to historically normal levels, there's way too many players chasing an ever shrinking pool of work. A lot of companies are hurting, but you'll never really know until the day that they can't pay you.
Just don't put all you eggs in one basket, so in case one stiffs you, you still have other work to fall back on to see you through.
In 1999, we had Intown Management file bankruptcy in a fraud case with their HUD contract. They owed us over $12,000 at the time of their bankruptcy filing, which was just for one month. They had always paid on time until the day that they filed bankruptcy and all of their assets were frozen. Long story short, it took 3 years to get 43¢ on the dollar, which in retrospect was probably not a bad outcome, all things considered. Intown only accounted for about 1/3 of our work at the time, so while it hurt, it didn't kill us. We lived to fight another day. Don't ever let anyone get too much into you that it will put you under if they default.