I think all four possible permutations are available: pl.+"of"+sing., sing.+"of"+sing., pl.+"of"+pl., and sing.+"of"+pl. If "constraint" is conceived as a non-count abstract noun, it may nonetheless operate in various ways, so that there are different types of financial constraint (pl.+"of"+sing.)--just as we may speak of AC and DC as different types of electricity, but the noun electricity can only be non-count. But maybe there is only one type of financial constraint operating in a particular case (sing.+"of"+sing.). If "constraint" is conceived as a count noun, and one regulation or contract provision constitutes one "financial constraint," there might be seventy-one of these constraints total, which might be intelligibly sorted into just five types, and thus you get five types of financial constraints (pl.+"of"+pl.); but in that situation, again, perhaps only one type of financial constraints operates in a given case (sing.+"of"+pl.). Maybe that last sounds and feels a little awkward, admittedly, but I do not think the same can be said of, say, "Carduelis is a genus of finches" (sing.+"of"+pl.).