It being worth it doesn't make it a real unique building. In most unique building cases, you want them built ASAP. Remove the Science and replace it with nothing and it's good, but nothing to change your plans over - if you have river heavy start, then the UB's power over the replacement is almost non-existent. That is NOT the case for Floating Gardens or Nilometer, which'd both outyield this one quite heavily. It is a nerf to a civ that doesn't need it. If you feel they're balanced now which you do admit, why nerf them for no reason? The RNG-based yields can just be changed to be reliable - worse than if in the optimal case, better than in the worst.
You ask why give this UB more yields when all I want is a change in how they're given so it stops being RNG-based, and I ask - why are we taking away yields from this UB when no one complained at it's strength and the nerf will be felt in every single one of our games, assuming you don't play a map without any lakes or tundra around? Even an average Sweden that doesn't start near Tundra's going to be losing tens of Science over it and prioritising expansion there, while being lame and weird, means you get a super-Science city. Of course, it's far weaker than if you started on Tundra or near a huge Lake, but you will feel it anyway, it'll just be something instead of carrying you. In both cases it'll feel wrong as it isn't what a warmongerer should be doing. Right now if I don't spand there, I'd definitely expand to decent-sized Tundra which typically also has enough sustaining resources just to get this Science, so yes - this nerf will be most assuredly felt, and it'll be very harsh.
On the other hand, if I start near Tundra now, I expand in the Tundra further knowing that I will be rewarded for doing so. Similar occurs with big lakes, or even smaller ones - as Sweden, they're prioritised and fought over. My gameplan changes because I can potentially receive more Science from this city than I will from my capital in late classical. You cannot honestly expect me to believe I, who sometimes play Sweden, won't feel the change when it was such a strong, UB-carrying feature, that I changed my decisions based on it's very existence, and while claiming that you also admit the civ is balanced as it is now. I felt the yields, I did things I wouldn't otherwise to some degree based on it every Sweden game, so your post just cannot resonate with me because it's wrong. What I hated was that where you start impacted it greatly - and just giving it stronger base yields while cutting out the terrain dependency, or changing it's features, would make it closer to okay. Right now it's too RNG-focused. It ranges from okay to bordering on broken, all depending on the map, and that isn't fun. There's too many map RNG lottery civs already, what with Siam (what CS you start near determines everything), Inca (self explanatory), Polynesia (to a degree), etc. While the Science per lake/Tundra is potentially mighty and it changes my gameplan, it doesn't do it in an experience-enriching fashion is what I'm trying to say.
To add to the previous points, I'll reiterate that the only complaints I remember in this thread were that the Swedish uniques are weak. There's very many civs in vanilla that, just like Sweden, nobody complains about being OP that got TWO good or very good uniques here, so why should Sweden in 4UC get two additional ones that would as a result be arguably worse than China's 4UC which people sometimes do complain about being OP? Even Songhai has superior 4UC uniques.
The UB isn't the weakest now, but after this nerf it'd be as it's only strong advantage would be being buildable outside of fresh water areas which, while nice, won't help on many starts and maps. You say your desired nerf to a civ you consider balanced wouldn't be felt on most maps, but Bastu as a whole wouldn't have any noticeable impact on just as many maps.The +3


is delayed and regular Baths have it meaning it's not impossible for Sweden to have access to it regularly, and you'll in most cases only get all this in medieval outside of super productive cities as you won't be getting an Amphitheatre/Garden/Temple before that (Garden's not available before Theology anyway).