What industries does it replace, in your opinion?
Using assemblers, I could entirely ignore Light Industry and produce consumer & luxury goods without using industry slots.
Light Industry is entirely obsolete with assemblers.
I could also supply heavy armaments, supplies, machinery, fuel and marines enough to meet demand and trigger the maintenance reduction.
I'm not using any AI cores, but I do have the Industrial Planning 2 skill, and have 4 colonies at sizes 6,5,5,4.
Assemblers have extremely minimal economy export (some commodities don't even have a single export unit up to size 4). Any vanilla industry will always export more than an assembler, and generate more income, especially since output is limited to one, maximum two commodities with a rare VPC - and that is before factoring in import cost for heavy machinery/required base commodities and upkeep.
I get that you want to give the assembler a passive income from the exports. But unless you can de-couple that from the triggering of the cost reduction, then it's way too much without the 'cost' of an industry slot.
That's why manufactories are fine, but assemblers are currently 'too good'.
The flip-side of this is the goods production. Which allows the player to make an
obscene amount of money just by having access to huge piles of fairly valuable stuff - which is fine, as the player must put some effort into converting it into money.
Imports don't cost anything though, so that can be ignored.