Quick comparison:
Ambrosian Republic:
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+10% National tax modifier
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+5% Morale of armies
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0/-0.025/-0.05 Monthly autonomy change
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+2 Number of states
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-30 Maximum absolutism
Oligarchic Republic (e.g., Florence and Lucca at game start):
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+5% National tax modifier
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-5% Stability cost modifier
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-0.00/-0.025/-0.05 Monthly autonomy change
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+2 Number of states
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-50 Maximum absolutism
So, Ambrosian gives +5% tax modifier and +5 morale modifier (helping fund your expansion and punch above your weight in combat), in exchange for extra stab cost (which, as a Republic usually operating at ~40-50% Republican Tradition, is already through the roof, so who cares). I ignore the two absolutism penalties, since (at least until Dharma drops) you will want to collapse to Republican Dictatorship and then Administrative Monarchy before the Age of Absolutism starts in both cases.
At tech 12, you can promote an Oligarchic Republic to an Administrative Republic (Switzerland and Siena at game start):
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-5 Years of separatism
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+10% Production efficiency
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-0.025/-0.05/-0.075 Monthly autonomy change
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+3 Number of states
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-30 Maximum absolutism
This is certainly competitive with Ambrosian Republic, as by the time tech 12 rolls around, you are spamming manufactories (so production efficiency is very welcome; it's only half of what you get when ahead of time in Admin Tech, but always there, which is nice). It's not obviously that much better than +10% tax income, but the reduced separatism and extra autonomy reduction are very nice. The major downside is 5 year election cycle, rather than 4, so average MP generation is a bit less than Oligarchic and Ambrosian. On balance, I would put Ambrosian and Administrative toughly on par, albeit with very different focuses. Of course, most Oligarchic Republics will only enjoy Administrative Republic status for about half a century, before both Ambrosian and Administrative will want to flip to some form of monarchy before absolutism hits.