The company has three drugs in phase 3, and another close to it - check out their pipeline page. Only KRN is likely to be a blockbuster, however, and Ultragenyx shares rights to the drug with its partner. That said, the other drugs are built on really solid science, had good clinical results so far, and serve desperate patient populations. That's a good mix.
I'd guess the stock is much higher a few years down the road. Analysts rarely ever get close to correct in guessing the size of drug pre-approval. If they can get multiple drugs approved, there's a good shot at least one will be a surprise breakout hit. If so, they will be worth a lot more than current market cap - if a bigger company doesn't buy them out sooner.
Dose response and signs of clinical benefit in phase 1 is pretty awesome. Infusion reaction in one patient is worrying however. That explains the sell off. With stock valued at around cash on books, that means you get a free option on the chance they figure out how to minimize adverse events, or they are manageable.
That sort of increase in sales should result in a greater increase in profit due to operating leverage. So even if revenue growth is 20%, that's sufficient to support a 30 PE - especially with other indications in pipeline that are essentially free options.