The thing I've seen with estimation practice last I researched it is, it works best if you can calibrate. That's something an established shop can do by extrapolating from their previous work, but it is also often as simple as "This other team took 7 months to do a similar thing. Therefore we will also take 7 months." An estimate like that is usually only wrong by days-to-weeks, since it encompasses all phases, eliminating the fudge-factor, unknown-unknowns and wishful-thinking aspects.
When it takes much longer, it's almost always due to design issues or political issues that create design issues. When the design is well-understood(and prototyping is hugely important to finishing design ASAP) the implementation goes smoothly. When stakeholders take turns stirring the pot to "make their mark", it goes haywire very quickly.
When it takes much longer, it's almost always due to design issues or political issues that create design issues. When the design is well-understood(and prototyping is hugely important to finishing design ASAP) the implementation goes smoothly. When stakeholders take turns stirring the pot to "make their mark", it goes haywire very quickly.