There are various ways to measure the success of a commercial product. Many of them are misleading. For example number of sales, or revenue, can be misleading if you don't know the profit margin. Selling a lot at a loss isn't success. And even profit numbers don't tell you the whole story, because $1 million of profit is a lot for a project that did cost you half a million, but is nothing for a project that cost you $100 million. So the good numbers you'd need are things like return on investment, or return on capital employed. And in many cases as an outsider you'll never be able to get hold of these numbers. So what remains to the observer is a perception of success which might or might not have anything to do with reality.
So kudos to Aventurine, makers of Darkfall, for their great managing of expectations. By running only one server, and limiting the number of people they let on that server, they effectively created a situation where it appears that people are banging on their door begging to be let in. The main complaint about Darkfall right now is the length of the time you need to wait in queue to get in. This creates a great impression of success.
There is no doubt that Warhammer Online has a lot more subscribers than Darkfall. But Mythic did exactly the opposite, opened up too many servers, and then Mark Jacobs made stupid comments on how WAR needed 500k subscribers to be successful, and how closing servers would be an utter failure. So when they reported first 300k subscribers, and then closing down 60% of their servers, the impression is one of failure.
Honestly I have no idea what the return on investment is on either Warhammer Online or Darkfall. I don't know how much these two projects did cost, I only have a faint idea of their revenues, and no idea of their profit margins. Darkfall took 7 years to develop, so maybe it was very expensive, but maybe they didn't have all that many people working on it, and it was very cheap. Do they really just need one server full of subscribers to survive? The only thing I do know is that there is a general perception of success surrounding Darkfall. Darkfall *seems* more successful than WAR, even if that might not be real.