Thing is, successfully operating in some of those environments as a military are things that may not require specialist skills, and there's certainly overlap between Forest and Jungle, they are terrain types you can specialize a unit into, by way of doctrine, training and equipment, and expect them to do better than a unit that isn't. Right now a unit with mountain and jungle capabilities does as well on a savanna or prairie as any other unit, despite most likely being prepared for a very different sort of combat than what such wide open fields and rolling hills permit. I want the ability to go 'these guys are absolutely trained and equipped with a doctrine designed to eke out every advantage possible on flatlands, and will ruin the day of those who were not'.
The ground forces game is already a numbers game, it's just a different numbers game because there are specializations for some types of terrain but not for others, and some conditions over others. Right now the best defense against an attack on a world where no capability modifier exists, like Temperate Forest for most species, is to simply exploit the fact that units with no capabilities are noticeably cheaper to construct with the same combat stats than units with capabilities, and to beat the opponent to death with your numbers advantage. A capability option would offer you a choice, to gamble on numbers, or to gamble on the capability being more effective than it's expensive once you account for the lower space lift requirements.
I first considered making Archipelago and Swamp the only Amphibious options, and I would love a greater variety of water based terrain types, but I added Rainforest and Jungle (and not their Mountain and Rift Valley sub-types) because that would leave a very niche capability and, AFAIK, most rainforests and jungles are very wet areas where travel by river is often the most convenient unless you can fly or are willing to spend a lot of effort on carving a road through the dense mass of trees, and if the dominant terrain isn't some variety of Mountain or Rift Valley most waterways should not have problems like rapids or waterfalls.
(Also, Forest and Jungle capabilities should be mutually exclusive but still have a lessened effect on the Jungle/Forest terrain type respectively.)
