To be clear I am not near unlocking this yet, and it's unlikely I would ever use it now since the advanced assembler is so OP.

The fusion assembler has 4 module slots and the nuclear assembling machine has 5. This makes the fusion assembler obsolete long before it is researched as far as I can see. This seems weirder to me as most of the unbalance I have seen so far is from the mashing together of mods, but these are both from the 248k mod right?

You're correct about them both being from 248k, but ATOM increases the number of module slots the nuclear assembling machine has. I don't know if the fusion assembler was deliberately left alone or it's just a case of no one thought of it.

"a case of no one thought of it"

I think it's that. I re-balanced the nuclear assembler at some point. I think I didn't do the same for the fusion one.

Having done some additional poking around since my last reply, I think the fusion assembler is fine as is. It's not the best choice for the "productivity above all" megabase meta, but it's high base speed makes it top-tier for anything that doesn't allow productivity, and the "doesn't require power" aspect might give it applications in coldstart setups.

Basically, it's a sidegrade, somewhat situational, but I think it's fine to have some of those.

I may have not unlocked the slowest recipes yet, but the advanced assembler, with the low tier wide area beacon, and merely tier 6 speed modules, seems fast enough. Point being, I'll never use it, because I can have prod 6 modules. And I'll only use the nuclear assembler for some science which allows 5 and doesn't work in the advanced assembler for some reason.

Doesn't mean it needs to be changed, but I'd still say it may be unbalanced.

For things which don't allow productivity, you should see what an advanced assembler with 6*tier 6 speed modules can do when beaconed by a wide area beacon with tier 6 speed modules.