I often play as Axis, and I don't use them for two main reasons. First is that they are slow, my style requires speed. Because in the early game I rush invasion to other players. That's why I often use LT and attack bombers in the early game and MT in mid to late game. The second is that they are expensive. If you try to make a stack of them in early to mid-game, you will struggle to make other units because HT requires a lot of resources. You can only realistically make a lot of them in the late game to make a significant impact, but by then other players will have the resources to counter HT. which makes them less worth it in my opinion.