FlexPDE

Recently found a PDE numerical solver for a wide range of problems,
including non-linear. FlexPDE allows for 1D, 2D and 3D problems, but I
can't understand the way it treats 3D models. To build a 3D model, one
needs to describe 2D-layers, "extruded" in 3-dimensions. Tutorial
samples show it's possible to build rather complex 3D-shapes, but still
I can't get, what is the problem to define purely 3D objects and solve
them in the same manner as 2D, by finite elements. Can anyone comment
on it, whether FlexPDE has some important limitations on 3D problems or
it's just fine, only gives flexibility? The reason of my question is
that there is a number of PDE solvers and I'd like to select one before
getting deep into details of each. This one seems to be very powerful,
with no drawbacks for 1D and 2D. But this "extrusion" is not clear to
me yet.

Thanks for you comments in advance.

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