NoSQL database vendor MongoDB just raised $80 million in additional funding in a round led by an unnamed sovereign wealth fund along with a handful of existing investors, including Goldman Sachs and NEA. The new funding, which values MongoDB at $1.6 billion, will be used in part to finance the company’s recent acquisition of WiredTiger, but there will be plenty of leftover funds once that task is completed. In an interview with GigaOM, MongoDB CEO Dev Ittycheria said the company raised three times what it originally planned due to overwhelming investor interest. In the interview, Ittycheria described this latest round of funding, the company’s seventh, as “opportunistic” and that MongoDB, while still planning to go public at some point, now doesn’t need to rush to file for an IPO.

While investor interest is generally a positive indicator of a company’s longterm prospects, it does not always signal that the business is currently thriving. In MongoDB’s case, it is becoming increasingly clear that the company has yet to arrive at a sustainable business model. There is no question that the NoSQL database category is on the right side of several secular trends, and it is understandable that investors want to get in on the action. But the challenge for MongoDB, as with competing open source NoSQL database companies, is to turn open source users into paying customers in large numbers. It isn’t clear yet just how many developers are willing to pay (or how much they are willing to pay) MongoDB for its technical & professional support services and management software. This latest funding round buys MongoDB more time to crack that nut before opening up its books to the public markets.

There is no question that MongoDB, the open source database, is the most popular of the NoSQL databases on the market. DB-Engines ranks MongoDB the fifth most popular database overall, well ahead of NoSQL competitors Cassandra and Redis. And MongoDB tops the 451 Group’s NoSQL LinkedIn Skills Index by a mile. But the popularity of MongoDB the database with developers is both a blessing and a curse for MongoDB the company. By all accounts, MongoDB the company has been extremely aggressive in its pricing strategy, which is understandable. The rule of thumb in the open source world is that just one out of 1,000 users becomes a paying customer to one or another commercial vendor. Vendors such as MongoDB obviously want to do whatever they can to improve that ratio, and aggressive pricing is one tactic to move the needle in a positive direction.

The problem, of course, is that unless the tactic is wildly successful and leads to massive scale, the company ends up taking in less revenue than the value of its software could command. Then CEO Max Schireson admitted this is the case at MongoDB in his keynote address at MongoDB World 2014 last June. The task ahead for Ittycheria, to put it bluntly, is to find a way to get developers to pay more for MongoDB’s proprietary software offerings, technical support services and professional services without alienating the very community whose support is in no small way responsible for the company’s sky-high valuation (Wikibon estimates MongoDB’s 2014 revenue at $46 million, meaning the company is valued at approximately 35-times lagging 12-month revenue). Having already set the pricing bar so low, this is no small task.

Adding to the challenge for MongoDB is that data management heavyweights such as IBM and Teradata, themselves threatened by the emergence of open source Big Data technologies, are increasingly developing both software and professional services offerings aimed at helping enterprises integrate and operate NoSQL technologies into their existing environments. While this will likely hasten enterprise adoption of NoSQL databases such as MongoDB, it potentially undercuts the value of the software and service offerings of start-ups like MongoDB (again, the company).

As stated, there is no question that the NoSQL database category is on the right side of a number of secular trends, including the emergence of the Internet of Things and the Industrial Internet. Wikibon believes that NoSQL databases will increasingly serve as the operational engines that, in conjunction with technologies such as Hadoop, make sense of all the data flowing from sensors and intelligently automate operational processes. That means companies that successfully leverage NoSQL technologies are going to create massive value for themselves, their investors and their customers. But there’s no guarantee that this value creation will extend to independent, open source NoSQL vendors. It might, but make no mistake, building a successful open source NoSQL company is going to be a long, hard slog.