If you're concerned about company viability, Zero is the best game in town. They have (by quite a bit) the highest sales, which has gone up every year, I believe for the life of the company (eleven years now?). That's a great sign given that other motorcycle manufacturers are seeing declines in sales, especially Harley, which is starting to flail around trying to stanch the bleeding. In fact someone just said on this forum somewhere that they can't find a 2018 Zero S to buy, because Zero sold out pretty early this year. That's got to be a good sign.
I don't believe Zero has tremendous amounts of VC money backing them, or huge grants of any sort, which means they're living hand to mouth without a big safety cushion, but it also keeps them highly motivated to sell product to keep the doors open. That's why their bang-for-the-buck (As Zarathustra says) is higher than the rest -- it has to be.
Compared by sales volume alone, the other makes are almost custom shops, boutiques at best. They may be viable because of large VC investments or some other large-scale backing, but it would be hard to call them full-on motorcycle manufacturers yet. Until Harley steps into the ring for real, I don't see that changing much.