It's difficult to know how best to address it. Almost any sort of penalty from miasma early-on before you have any way of dealing with it would be punishing.
I guess one way to deal with it would be to move the 'remove miasma' worker ability earlier in the tech tree to something like planetary survey, which you can completely ignore if you're land-locked.
I guess one sensible approach would be to tie it into health? Something like -0.2 health per miasma tile within your borders, and -0.4 health if worked? Then the -10 health penalty could be an extra 20% penalty from miasma, and -20 health could be 40% penalty from miasma. Little tricky having a health penalty that compounds on top of itself, though. But this would help to constrain aggressive early expansion, while not hurting those who go slower with workers nearly as much.
Miasma repulsors probably need to be nerfed a bit - it seems at the moment that in 10 turns they will remove miasma from 19 tiles? Could be bumped up to 15 turns, and maybe even give a -2 health penalty for any city within the radius for the duration, because pumping all those cleaning agents down willy-nilly can't be good for the people right? Would make clearing miasma harder and encourage to use workers to do so instead. Given the opportunity cost of having miasmic repulsors above your cities instead of solar collectors, increasing the duration to 15 turns is an additional nerf.
