Water based reactors are actually not bad at all because water is a moderator and if there is less water, because of heat causing steam bubbles, the moderation effect is lower and the reactivity goes down, causing less heat. This makes water a safer coolant than other coolants.
But yeah, it would be limited to slow neutron reactors, but I don’t see why those shouldn’t be able to scale into the late game, with (pressurized) heavy water, chrome heat vents in a chrome reactor, especially when their fuel is more readily available.
Some ideas for the coolant types:
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Water/heavy water: Produces steam based on cooled heat. Heat vents act as moderators, moderation based on water-steam ration in the reactor. Neutrons doubled by moderation are useless for breeding. Heat vents have a very high heat transfer rate, but a low heat capacity.
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Pressurized water/heavy water: Produces super-heated pressurized water based on temperature (can be heat exchanged or turned into steam), heat vents act as moderators with a constant moderation value. Heat vents have a very very high heat transfer rate, but a low heat capacity.
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Air: Produces heat energy on the top of the reactor based on fuel rod energy. No special properties. Heat vents have a low heat capacity and medium heat transfer rate
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Gas: Produces heat energy based on the fuel rod energy on the top of the reactor and heated gas based on the cooled heat (can be heat exchanged or directly power a turbine). Heat vents have a medium heat capacity and medium heat transfer rate.
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Liquid metal: Produces super-heated liquid metal based on fuel rod energy (must be heat exchanged). Heat vents act as component heat vents. Heat vents have a very high heat capacity and medium heat transfer rate. Allows for highest core temperature.
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Molten salt: Produces hot molten salt (must be heat exchanged) based on fuel rod energy and cooled heat (additive). The hotter the core of the reactor, the less energy and heat adjacent fuel cells produce. Heat vents have a high capacity and high heat transfer rate.
This makes air reactors the most easily usable, especially early game, but definitely the weakest. Gas reactors are a straight upgrade, also directly producing heat, but also super-heated gas.
Liquid metal reactors are best for fast-neutron reactors, breeders, allowing for very high temperatures and being not too hard to cool, but are comparatively bad for energy production.
Water/Pressurized Water reactors are good energy producers, but terrible breeders.
Molten salt are very safe and good energy producers, which are more versatile than water reactors.
Obviously this is not all entirely realistic (“fuel rod energy”, lol, i.e, the IC2 electricity output) and greatly abstracted, but I think going too realistic can very quickly get out of hand (Sorry Xenon). Especially fuels could use some creative liberty to make them more distinct.