How flexible is the 'My Football Now' game engine?

Not sure exactly what you mean by 'flexible'?

I play for years, in addition to FOF, in a fun browser american football management simulation.

Colleagues GMs have plenty of suggestions.
Unfortunately, the developer says every now and then, that this or that proposal is not easy, or not at all to realize with the used engine.

In that regard it is very flexible - there hasn't been a suggestion yet that is too difficult to implement, it's just a matter of priorities and development time.