Even though Pokemon GO was first released apart of the Sixth Generation of Pokemon, you can only find 1st generation pokemon, 2nd Generation Pokemon, and now 3rd Generation Pokemon within the game. At first, you may think that the game is going to be a breeze because of this small fact. But even though all you must do is catch a grand total of 386 Pokemon to fully complete your National Pokedex - minus, of course, Region Exclusives (until Trading has been incorporated within the game) and the numerous Legendary Pokemon that are still unreleased - this task is going to be anything but easy, especially with the concept of catching in Pokemon GO.
First, you must walk/travel everywhere. You may think this isn't too hard, you may even think you like the exercise at first, but after miles upon miles of walking and still finding virtually only a fraction of your pokedex, the concept of the game and your desire to complete the Pokedex will get old pretty darn fast. Add this to the complex ways of training and evolution, all practically done by continuously capturing Pokemon and defeating raid battles, and you might have to force yourself at times to get up and "GO".
On top of all of this, there is no possible way of tracking every single Pokemon in your pokedex. In the Mainstream Pokemon Games, you can at least use your Pokedex and maps to find the majority of the Pokemon you need. But in Pokemon GO, unfortunately, you don't have this privilege. All you have is a list of nearby Pokemon, which only shows up to nine Pokemon at any one given time, and some of those nine Pokemon might be the same as others on the list or ones you've already previously caught. It's a behemoth of a task to find every single Pokemon in Pokemon GO - it's almost impossible since Trading has yet to come - even if it is just the first three original Generations that you need to find.