I reckon the hardest thing in a fugue is puting the melody in all the voices without it getting boring at the start. Especially if it's more than four part, hearing the same thing over and over at the beggining could start to get tiring if you don't mask it well enough.
When you write a fugue, just remember that you don't have to have the whole piece in fugue. There's no compositional ref who's going to disqualify you if you don't follow the form exactly. The form is just meant as a starting point, as a base. Feel free to move around a bit.