Sure that it is possible to know enough to any purpose as long as we're aware that the lack of complete knowledge harbors the eternal promise of surprizes. When the surprize hits it becomes the really wicked problem. The only solution to this eternal lack of knowledge is the eternal learning - and the Systems Integrators have to excel in learning just for their professional survival.
The Systems Integrators always start with the lack of knowledge - for all things equal they usually are not those who have designed this vile creature to be integrated. More that that - there are always parts on the bottom of the hierarchy (the Second Secret nonwithstanding) that are not created by us but are bought from somebody that may not reveal the whole bunch of their properties. Or even the natural things that for them the lack of knowledge is inherent.
The lack of knowledge is not an excuse for Systems Integrators. They have to tell the system's story (the First Secret) and to deal with the networked nature of the system (the Second Secret). So the Third Secret of Systems Integration may be formulated as "There is always missing knowledge that th SI has to discover in order to succeed".