turning an AI into a tributary is better than automating a sector, they're both controlled by the ai anyway and ai has cheats so its stronger, especially taking into account branch offices and holdings
This is simply incorrect.
1. vassal AIs do not have cheats. they crash and burn into bankruptcy. the moment you vassalize them is the moment their death spiral starts due to losing their cheats
2. even if they still had the cheats, they would still be very inferior in terms of using fleets
3. vastly more diplomatic weight from integrating them compared to just keeping them as vassals with limited diplomacy (because now they get all your multiplicative bonuses)
4. the increase in unity production largely mitigated by the increase in unity cost (it still slows you down a little though... assuming you focus on science over unity)
5. the increase in research production overtakes the increase in cost to research tech. so it is still a net gain to your tech up speed.
6. they might have some cool things you could grab, such as dig sites, damaged megastructures, or special worlds (ecu and relic). You could claim them individually, but it is very expensive to do so and has to be done before you vassalize them.
7. you get a big injections of fleet capacity via all their anchorage starbases. (scrap the rest of their starbases). And sometimes even some useful starships.
8. tribute is split into 4 different categories (science, base resources, adv resources, strategic resources) and raised in small 15% increments, and you must raise it bit by bit over many renegotiations, paying ruinous amounts of influence every time. Much better to take everything all at once.
9. as a tributary, it will eventually on its own against your will just transform into a useless bulwark. where suddenly you are forced to pay THEM basic resources for the privilege's of them building useless outdated starbases and fleets. And there is nothing you can do to stop it.
It is vastly more profitable to integrate a vassal than to keep it as a tributary.
However, do keep a few vassals with the middle option for expansion as a source of influence (as you "tax" their influence)
The biggest drawback, really. is that they often fail to build space roads, and for some inexplicable reason those cost influence.