Well, the short answer is that a one brick/two brick forge is better than no forge. It certainly is cheap and easy to build.
To answer your specific question about uneven heating...yes, that can be a problem. The blade must be watched, and perhaps moved in and out slowly to get a more even heating. For forging, a one bricker can work just fine. For HT it will take careful control of the amount of flame, the distance in or out of the port hole (where the torch enters the forge) and even movement of the blade.
Letting the brick forge run for several minutes to fully heat up the brick interior is a good idea to get a more even heat. This is called pre-heat, or soaking the oven, and is a good practice with any type of forge or oven.
Final sugestion...use a Bernzomatic JT-7 torch.