A trailer is likely to be 10 pounds heavier than panniers. TEN POUNDS that you have to drag up every hill!!
That ought to be enough right there, but then also consider that you have to ship it if you want to fly to or from your tour.
To over come those two disadvantages you have to really like some of the advantages.
I own a trailer and it just hangs on the basement wall. Want to buy it? I will give you a good price