The original 1997 Earth 2140 was very, very advanced for its time; the jewel in the crown was a variable and adaptive AI that could be more challenging and unpredictably cunning than even some human players.
The Steam/HD version lacks this AI, or any AI at all, and most of the fun has gone out of the game because of it. "Unplayable and boring" is a harsh but accurate description that's frequent on the Steam forums.
The original game is an underrated and magnificent game with great unit variability and faction balance. The missions were definitely challenging, even for seasoned players, and the adaptive AI really kept a player on their toes. The combat mechanics, friendly AI "Generals", resource/production nuances, and other features were well polished and supported a variety of strategic play styles.
The HD version includes two Mission Packs, each with a selection of well-made missions and short campaigns, but without an AI for your opponents or friendly generals the missions just become a slog of whittling down the auto-produced horde milling about the enemy base.
My recommendation is to go buy an old CD and install it on a machine that will run it instead of buying the Steam/HD version; This was a great game, and it needs to be played in a version that will do it justice.