We do not yet know whether the brain is "understandable" in the sense that we accept now. If the brain operates mostly on the basis of several dozen key principles that can be expressed mathematically or with some other formalism, then it will be understandable in the conventional sense. On the other hand, if it is in fact tens of thousands of formulas with no apparent organizing principle, then we probably won't say we understand it in the sense we accept now--but we may be increasingly willing to hand over the details to computers.
There's every reason to believe that the brain is understandable in the sense that we can predict what it will do given adequate knowledge of its present state and inputs over time. Some aspects may be chaotic or random, but we know how to characterize such things. But what's not clear is whether "mind" will be like "life"; we generally have a sense that we understand what "life" is since we can break it down into a few basic principles--replication, metabolism, etc.--which in turn can be broken down into a few basic principles and so on. Even though maintaining a living organism is incredibly complicated and we are missing very many of the details, we're mostly comfortable saying that we understand what life is and how something is alive vs. not alive. There is no particular reason why the brain must be understandable in the same kind of hierarchical fashion, however. We can even build things that we can't understand. For instance, rather frighteningly, nobody really understands any more how the electrical grid in the United States behaves in the face of a large failure; this is because of exactly the feared brain-problem: thousands of local decisions about how to handle failure and thousands of local structures contribute to the stability or lack thereof of the whole system.
But in the longer run--assuming we maintain an advanced civilization--we don't really need to worry about whether we understand correctly or whether we plus computers together understand (in the high-fidelity-of-prediction sense).