hydro and nuclear need the very same turbine tech, and actually turbine are the most hi-tech part of the game, reactors demand delicate construction and testing, but behind that are not much complex.
Oh, sure, IF you have a vast set of mountains and much water, let's say Norway, developing hydro is obvious. Just a bit more populated countries, let's say Swiss do want both simply because one offer good power for most of the year but not enough safety to relay on and so having two systems is better than one. If you have no suitable areas, let's say Mongolia, and you do not have the tech for nuclear... Than they are both unfashionable.
That's the practical "fashionable" or "unfashionable" part. Anything you can have, is fashionable if gives you something useful, anything who can gives you something useful but you can't have........
They are both build with the same industrial process, demand similar alloys, similar CFD calculus etc. Witch means: if you can build one for hydro you can build one for nuclear. If you can't for nuclear than you can't equally for hydro and vice versa.
That's why they are a big business for very few developed countries supply chain. Do NEVER forget that anything at a certain scale is a matter of know how, supply chain and raw materials. Formally knowledge is "open in science" but that's just PR, in the modern world almost without public research and public universities knowledge is power and is in private hands to a point of being a danger for the society at a whole.
They do not, in fact, demand similar alloys. A hydro power turbine operates with liquid water, a steam turbine with superheated, corrosive steam.
A Pelton wheel is fundamentally different in every detail from a steam turbine. Useful Pelton wheels have been made in village smitheries. They are readily mass-produced, and last many decades without need for service.
Oh, sure, IF you have a vast set of mountains and much water, let's say Norway, developing hydro is obvious. Just a bit more populated countries, let's say Swiss do want both simply because one offer good power for most of the year but not enough safety to relay on and so having two systems is better than one. If you have no suitable areas, let's say Mongolia, and you do not have the tech for nuclear... Than they are both unfashionable.
That's the practical "fashionable" or "unfashionable" part. Anything you can have, is fashionable if gives you something useful, anything who can gives you something useful but you can't have........