Yes, intel is largely guesswork. How else can we explain how an alphabet soup of government intelligence agencies, both here and abroad, failed to predict things like the collapse of the Warsaw Pact and the Soviet Union?

Unlike nature, governments actively work to deceive one another. Human sources are often suspect. "National Technical Means" - the official term for things like reconnaissance satellites, show far from a perfect picture. Their orbits are predictable so technically astute opponents know when they have to hide what they're doing. The other side is doing their best to hide their capabilities and intentions. Everything is subject to interpretation by analysts, some of whom have agendas. There are multiple competiting agencies (CIA, NSA, DIA, NRO, NIMA, to name just a few) and multiple departments within each of the agencies - and they often refuse to share information with one another due to compartmentalization and bureaucratic turf wars. For example, SCI stands for Sensitive Compartmented Information - a level above Top Secret where only people cleared for a particular compartment have access to the info. I'm cleared for multiple compartments and it still isn't enough to do my job, and there is at least one level above SCI.