Miniguns, such as the M134, are known for their high rate of fire rather than pinpoint accuracy. They can fire thousands of rounds per minute, which makes them effective for suppressive fire and covering large areas. However, this high rate of fire can also lead to a spread of bullets, reducing precision.
They are designed to hit targets within a certain area rather than a specific point. The accuracy can be influenced by factors such as the mounting platform, the stability of the shooter or vehicle, and the distance to the target.
For specific applications like close air support or vehicle-mounted operations, the minigun’s accuracy is sufficient to achieve its intended purpose. However, for tasks requiring precise targeting, other firearms would be more suitable.
In T2 he was using it suppress fire in certain areas, and move the police away so they aren't a threat, that's something the gun is designed to do. In Terminator Zero, he's trying to hit a single moving target, constantly moving and dodging, thats not something the gun is designed to do.
It's much easier to shoot at something and miss then to actually hit a moving target, especially with a weapon made to suppress fire, and spray and pray. Your example doesn't prove they are a expert marksman because he missed it shows he had the right weapon for the job he needed to do in T2, but in TZero he had the wrong weapon for what he was trying to do.