No assult tank for the Atreides, and i hope you can only buy your units from the Starport. For the Ordos, there is no concept of cowardice. Its more a matter of efficiency. The Atreides 'tank' unit is the Mongoose. You'll learn to appreciate them very early on.
Every commander knows, above all, that knowledge is power.
Keep an eye on everything that happens, and intercept attacks beforehand, by way of a quick LasTank run, or any manner of quick attack. If the oncoming attack is too much, hit it, run, then attack while near base defences, so they have to contend with both at the same time, if you use the artilliery based units to attack the base defences, your assault units have to stand back, allowing quick hit and runs on the Artilliery. If you move your Assault vehicles in, they get battered by the combined force of the base defences and the standing army, and also allow for the possibility of a small force circling around and taking the Artilliery from the back. The above, does, however, depend on the range of the aforementioned artilliery. Granted, if your forces are tooo numerous, then that would prove fruitless, but anyone that allows their opponent to gain a clear economical advantage is asking for a loss.