So my neighbors umbreon and my sylveon ended up mating and now I have 3 baby eevees. The only thing is, one is melanistic. Now I’ve heard albino eeveelutions and other albino Pokémon are very temperamental and neurotic. Is there any behavioral issues I should be worried about with melanistic Pokémon?

Generally speaking, no. As mentioned in an earlier post, while albinism is linked to certain health conditions (meaning albino pokémon are at a higher risk of being actually, physically sick—but do keep in mind that this is a risk and is not true for all albino pokémon by default), melanism is not linked to any particular health issue. Thus, a melanistic pokémon may act in the same way as a brown-coated eevee, albeit a melanistic pokémon may be hardier than its brown-furred cousins.