i only want to say one thing about why UFO:E is a broken game -- the design decision to have immortal soldiers. no, i'm not talking about immortal soldiers removing tension or anything like that. the decision to have immortal soldiers has further game mechanic implications that usually go unnoticed.
in relation to immortal soldiers, we have no encumbrance. instead of an encumbrance system, chaos concepts used a strength system -- each new weapon up the tech tree took more strength to carry than the last weapon. UFO:E had a lot of weapons -- more than 20, if i remember correctly. with immortal soldiers, a strength system works because soldiers never die and they will always be able to use the latest tech.
but when people tried to mod in soldier deaths, this problem came up. new soldiers were utterly useless because they could not carry the latest tech. the aliens in the game also scaled up as the player researched new weapons, so older weapons would become completely obsolete.
i see this as a side effect of immortal soldiers -- i'm sure it was easier to implement a strength system than to create an encumbrance model, and the developers could do it because of the immortal soldier design.
in addition to this is the fact that you could not pick up an "incapacitated" (the game's way of "killing" soldiers in mission -- if the mission was completed successfully, those soldiers would be brought back to base to be revived in the hospital) soldier in the tactical game. so if things were going badly, you'd have to leave soldiers behind to actually die (the only way soldiers could die in UFO:E). any replacements you received (which the game sent to you, one at a time, about every other month) would be baseline rookies. if you were far along in the game, the loss of a soldier would be game ending because the new recruits would not have the strength to carry the latest weapons required to combat the scaled up aliens.
and this, ultimately, destroyed why chaos concepts said they put in immortal soldiers -- to prevent players from reloading the game every turn if something went wrong. as the game is, you have to reload if you lose a veteran -- rookies are useless towards the end of the game.