Career prospects for Math Finance researcher
There's no question that mathematical finance has a smaller market for academics than, say algebraic geometry.  (However, if you can pitch yourself as being more broadly interested in, for example, stochastic processes, that's almost as big a market.)

Small markets are subject to more variance on both the supply and demand sides.  You could apply for jobs in a year when no one is actually interested in hiring in mathematical finance.

On the other hand, if comparisons could actually be made across fields, the best person in mathematical finance one year could be equivalent to anywhere from the best to the 10th best person in algebraic geometry that year.  If someone is really looking to hire in mathematical finance, that person will probably be hired.  In algebraic geometry, while the best algebraic geometer on the tenure-track job market in a given year is almost sure to be hired somewhere, the 10th best should be looking at other options, and even the 5th best should be quite worried.