"would have been" is the unreal past form. "would be" is the unreal present form*.
Because this statement is in the past, and that they did not implement serial updating, the idea of implementing it in the past is imaginary. As such, we want to use the unreal past.
To make it a bit clearer, we can put it into a classic "If A, then B" form.
If they had chosen serial updating, it would have been too restrictive.
If we were to use "would be" it would be in the present (like this sentence). For example,
If we were to use serial updating, it would be too restrictive.
Now, this is talking about the future. We don't plan to use serial updating, so we talk about it in a way that tells us that we're just imagining something that isn't (and won't be) true.
(*In some cases "would be" can be the past real form, but only when telling stories or reporting speech.)