During my industrial PhD, I created an Object-Oriented Programming (OOP) framework for Large Scale Air-Pollution (LSAP) simulations.

The OOP framework I created was based on Petrov-Galerkin FEM. (Both proper 2D and "layered" 3D.)

Before my PhD work, the people I worked with (worked for) used spectral methods and Alternate-direction FEM (i.e. using 1D to approximate 2D.)

In some conferences and interviews certain scientists would tell me that programming FEM is easy (for LSAP.) I always kind of agree and ask how many times they have done it. (For LSAP or anything else.) I was not getting an answer from those scientists...

Applying FEM to real-life problems can involve the resolving of quite a lot of "little" practical and theoretical gotchas, bugs, etc.