Annexing Minors Makes the Game Too Easy...

[BlueTemplar]

[...]
Farmers can be present on many planets, atmospheric farming is not that rare. Also the hostility of the planet is pretty much irrelevant for difficulty purposes, noone cares about envirosuits.
[...]

I have trouble to believe that,
because Envirosuit+Slugthrower is 2x Metal and 3xIP cost of naked+Slugthrower,
and Envirosuit+Machinegun is 1.5x Metal and 2xIP cost of naked+Machinegun,
it will (or at least should) greatly affect the number of human infantry (and other troops ?) everyone starts with (including Minors and maybe even Non-Aligned), how the militia replenishes (Equipment pool vs Manpower pool), and of course their protection (HP) : 100 x design rolls vs 50.
This will (or should, logically) have an impact on relative power of human infantry vs all the other troops, including later of militia vs newly modeled troops, since militia lags behind for quite a while...
(Similar, but less extreme for Filters and/or Thermosuit.)

[GazBot]

Heres some ideas to make Dipplo Annex more difficult....

1. PLay on hard levers (hardest preferred....) this increases all the rolls you need...
2. tick the extreme violence option....this means you will 99% only get raiders and fanatics as minor regimes...these guys are always hardest to turn diplomatically.

Even doing the above it can still be done...

You need to use Shadow diplomat when going for protectorate and client...ideally after making a peace treaty always respond positive when they make requests...this bumps up your relation....ideally do this prior to making them protectorate and client.

You have to be looking to increase relation right from the start...this makes all the subsequent skill rolls easier....

Then when you unify you need an advisor with a high oratory skill value attached to your foreign council....this is how yo umaximise your chances....or someone with high intimidate if your going to annex them....

[Xxzard]

Following up on this thread as of 1.21g beta,

The AI on Hard is now very good at diplo annexing. In my 3 most recent games (tech 3, city state militia only start) a lot of minors are AI protectorates by turn 2 or 3, and fully integrated by turn 10. I am starting to adjust my starts to focus a lot more on diplo if starting next to possibly friendly minors.

Edit as of 1.21h beta: Patch notes say frequency decreased, so the effect may not be quite so extreme now.

[Voker57]

This is on my radar. I don't consider it too easy when playing PBEM since everybody could go for the diplomatic route to annex the Farmers, but against AI it does indeed cause a bit of an inbalance as the AI won't even try annexing (it will fight instead). This is something I want to add in the near to medium future. (together with a bunch of other AI improvements)

It is still an issue with PBEM because one player can end up positioned with all farmers, unbalancing the start, and generally it's a no-brainer to annex these. Because of that PBEM are either played with Severe Violence or no diploannexation house rule.