They Are Billions

I remember this next one being quite popular right when it came out, and I, too, liked the premise. There are lots of zombie survival games that make you run around, avoid the zombies, maybe kill some here and there, but They Are Billions actually goes and lets you face the hordes. Literally thousands and tens of thousands of zombies at once, and you take the assault head on. Truly a fantastic idea. There is of course the usual disclaimer that I'm an idiot for yet again playing an RTS when I don't really like the genre, but sometimes you just have to take what you can get.

Looking at the campaign side of things, They Are Billions is composed of a series of levels, each with an objective mainly consisting of killing zombies and staying alive. Often there is also a population requirement and/or a time limit. For what grander purpose, I can not tell, as you need population anyways to get workers for the resource collection buildings (food, wood, stone, iron), soldiers, and well, why not let people take their time playing if they so desire and are not otherwise overrun by the hordes that spawn every so often and charge your base? The gameplay is roughly split between the combat and the colony management aspects, as a lot of effort goes into making sure you have all the necessary production set up to build units and defenses. In the end, probably only 5-10% of your buildings will be military-related.
The campaign also includes a technology tree which allows you to unlock more impressive units and stronger buildings as you progress. And then there's another mission type entirely, where you control a single unit, clearing out smaller levels, without the building mechanics.

Honestly, for the first few missions and hours, I really liked this game. But the more I played, the more I noticed problems started to crop up, both big and small. The game was kind of slow, partially owing that to how it wasn't entirely focused on the best bit - combat, instead forcing you to build up your economy first. Once you got that going, the progression speed was more related to how fast you could build (unless you ran out of uninfected room). They could have reduced the number of houses and food gathering places needed, as well as removed the energy distribution grid idea entirely. Further, building was a mess, as there were many requirements about what could be built where, and which buildings could be built how close to each other. The restrictions were entirely unintuitive, and neither were there any visual indicators where stuff could be built, leading to trial and error, which was also a big time sink if designing an effective base.
All this detracts from the sweet experience of killing buttloads of zombies, but worst of all, and what was the final straw... Once you've finished all the boring setting up and can use your military to finally start more agressively fighting the zombies, your chance of failure starts rapidly increasing. If you make a slip up and get your defenses breached, it's game over, and you have to start from the beginning, doing the tedious setup yet again.
As briefly mentioned, there were some other weird choices made by the developers, such as having odd population and time limits on missions, or the whole concept of "hero missions", which are just a slog through a building, playing pixel hunt with indiscernible pickups that you desperately need to advance your tech tree. Lots of other quality of life things were missing as well, like more sophisticated commands for units, such as formations.

Overall, I believe They Are Billions is a great idea, trying to fulfill the fantasy of slaughtering an insane number of enemies, but numerous questionable design decisions and lack of quality of life functionality are a dealbreaker. I can't force myself to go through the lengthy boring bits again and again, just to get to the good ones, and unless you got a lot of tolerance for repeating non-challenging tasks and a lot of time on your hands, I can't really recommend this RTS to you. As an alternative, maybe try one of the Creeper World games.