Any risks/disadvantages of using MacBook Pro as a desktop?
You really hasn't stated you workflow but here is my thought:

The main disadvantage to me in this scenario is total cost by which I understand less powerful CPU/smaller screen for more money and also higher risk of getting into problems with GPU since it is the most failure prone component in notebooks as we see for years. And don't forget that a cup of coffee can cost as much as new Logic Board, Topcase and Labor.

For this scenario why don't you look at Mac Mini (preferably older 2012 Server QuadCore one with Intel HD4000) and as small monitor as you like? Or a smaller iMac with integrated Iris Pro and Quad i5? If you are so keen on Keyboard and trackpad being together look at Magic Wand and that trackpad is bigger than in Macbook.