When these policies and guidelines are strictly observed, the advantages of Tasers maximized and their shortcomings are all but eliminated.

Should are be taken out of the sentence? or should it be were all but eliminated?

You need to add "...are maximized, and their...."

Or you could revise it one of these ways:

When these policies and guidelines are strictly observed, the shortcomings of Tasers are all but eliminated.

When these policies and guidelines are strictly observed, the shortcomings of Tasers are minimized.

I hope this helps. Thanks for asking.

I think it's:

When these policies and guidelines are strictly observed, the advantages of Tasers maximized and their shortcomings nearly eliminated.

Am I correct?

Put "are" before maximized....

In this sentence, the use of "are" is correct. "Are" is the present tense of the verb "to be," which matches the tense of the main verb "observed" in the sentence. The phrase "are all but eliminated" means that the shortcomings of Tasers are almost completely eliminated when the policies and guidelines are strictly observed. The word "but" is used as a conjunction, implying that the elimination of the shortcomings is the only exception to the maximization of the advantages. Therefore, "are all but eliminated" accurately conveys this meaning.