The two fold effect of landing in tailwind when there is a stronger (tail) wind a little higher is also the overshoot you will get from a decreasing tailwind.
Tailwinds on final are just uncomfortable. Different thrust settings, different attitude and therefore different picture, different perspective as you reach the flare point etc etc. Combine all that even five knots of overshoot shear and it can all turn messy in a hurry.
5 knots is my personal limit at touchdown but only on certain runways (IE about 3000 metres of runway and visual) and all other things considered of course. Wind also has to be less than 10 knots tail at 500'. I developed that rule after flying into DN about midday in Summer. Seabreeze was just coming in and so had 290/15 all the way to about 50' when it changed to 110/2. Floated a LLLLOOOOONNNNNNGGGGG way down 11. No one else had any clue that it was going on. They changed runways after our arrival.
Good question though and worthy of discussion.