Invited to write a review on a field that neither my Prof nor I are experts in? As a PhD student nearing the end of their degree my Prof invited me to write a review with him. A decently well known journal had asked us to write a review on a topic that is only tangentially related to our labs area of study. My prof thought this was a great idea since it "guarantees a publication" for the lab and would help get me a first author publication, but I was extremely hesitant at first because I have 2 parallel experiments running and neither the prof nor I were experts in the field. At first I was super optimistic that we could write the review but the more I researched the more realized just how in over my head I was.

Thing is that my Prof. said other grad students in our lab also wrote reviews at similar stages in their PhD degree but looking at their reviews it was clear that the their topic was either something they were actually directly studying, or something that my Prof. was an expert in. Both of which is not true for the review I have to write.

With only a year left in my PhD, I am really worried if this review is a massive waste of time, since all my experiments will need to be put on hold while i read countless numbers of papers in a field that I'm not really an expert in. Should I tell my Prof. that this review is a bad idea and cancel it?
Your prof is thinking long term. You are thinking short term. Both of those are appropriate. But don't think too short term. Within a year you will want your horizon to expand and both the good opinion of your advisor and a publication will help. 

But, I didn't read you as implying that this must become the first priority over a period of time. I don't see why your experiments need to be put on hold while you read papers (probably fewer than countless). 

But, if your advisor wants to do this as a joint project ("write a review with him"), then you should be able to manage it as long as it isn't all dumped on you. I suggest you set a schedule to work together on it and lay out a plan. 

And maybe the new field will prove interesting and even valuable in your future. 

Of all these things, I think the good opinion of your advisor is the most important to manage. If you can do that somehow other than agreeing, then there is little downside. But there might be upside to agreeing. Your call, of course.