" the powers would only be invoked for “serious crimes” involving sentences of three years or greater."
And we know that they will stick to those limitations, because?
If nobody is allowed to know what they are doing because any whistle-blower gets a 5 year sentence, then they will misuse it. Any time some group gets completely unsupervised power, it gets misused.
This is well known - the Snowden revelations showed that security agencies go way past what they are theoretically allowed to do as a matter of routine every day operation.