All types of improvements have their drawbacks. However, you have to use them in some cases, because providing the bonus via a technology makes no sense, is impossible (some bonuses are planetary only) or is simply OP.
In case of the Drengin, I decided to add a +10% Loyalty-bonus to Xeno Propaganda. They already have enough 1pp improvements. Adding another one would make things only worse. A SP would have been too similar to the Propaganda Center. Not to mention all the other drawbacks a SP would bring.
I'm currently testing my changes, and just realised why I took a break. The Drengin's research pattern is completely nonsensical. They go for everything, except the techs they could really use, like Slave Pits, Diabolical Research or Cutthroat Trading. I have no idea if this is due to one of my changes, or because I'm using a different testing-method.
One more thing I noticed is, that I forgot the Navigation Centers when I was "fixing" the 1pps. In other words, it can still happen, that the Arceans can not rebuild the NC, if they have upgraded it, and it got later destroyed. I have three ideas on how to fix this, but all have some drawbacks:
1. Remove the Advanced Navigation Center improvement and add the speed-bonus to the tech. The drawback here is that a +10% bonus seems pretty low for a cost of 3500, but that was already the case before. Plus, one tile seems like a pretty high price to pay for just +1 move. To fix the former, I could either move the +10% bonus from Interstellar Navigation to ANC, so it provides a total of +20%, or reduce the tech-cost for ANC. No idea about the latter though.
2. Remove the NC. This would have the drawback, that the Arceans would be much slower in the beginning. Not to mention, that there is the likelihood that the Arceans don't have any tiles left by the time they research ANC.
3. Remove the ANC improvement and add a +20% speed-bonus to the tech, remove the speed-bonus from Interstellar Navigation and increase the move-bonus of the NC to +2. This is a variation of the first idea. The drawbacks here are, that only the ships build on a planet with the NC get a speed-bonus in the beginning, and that the Arceans become overall much faster. I already gave them a boost by removing their speed-penalty. Giving them yet another boost seems like overkill. The Arceans are supposed to be slow.
One last thing: I reworked the maintenance-costs for Labs and Factories again. Since I began working on my mod, I alternated between a slowly increasing maintenance-cost (as it is in the current version) and a fixed maintenance-cost. Now, I went back to the fixed cost and think I'll stay there. The AI has far too many problems coping with the increasing maintenance. My current values are:
Slave Pits 1bc
Slavelings Labs 2bc
Research Collectives 4bc
The Thalan improvements are unchanged.
If I manage to tame the Drengin and nothing major comes up, I'll release the new version this weekend.