FEniCS has been used for very large problems, Scalable computation of thermomechanical turbomachinery problems - ScienceDirect (3.3 billion dofs in this paper).
We have also tested it for large problems (up to ~200 billion cells, but I don’t have preprint).
We also perform performance tests GitHub - FEniCS/performance-test: Mini App for FEniCSx performance testing · GitHub
Here is a plot made by @Chris_Richardson a few years ago illustrating the scalability:
In general, us as developers don’t like voicing our opinions on other software, that will always be biased, as mentioned in: FEniCS vs. MOOSE - #3 by garth
Of course, end users are free to comment on pros or cons (but might be biased since you are asking on the FEniCS forum).
I would suggest looking at the tutorials and demos