First con I can think of is ..... you've already passed whatever you're looking at on the screen. That would include 2d/DI/SI image units. For those units that have 360deg scanning capabilities, you'd have to understand that you are "actually" X number of feet closer to the images you're seeing on the screen. For Forward Facing scanners ... you'd have to have a way to turn the tdx from your position, plus you'd have to allow for the distance between you and the tdx when measuring the distance to your target IF the target is directly in front of the boat, if the tdx is stationary. IF the tdx is movable by some mechanism you control, you would still have to account for the difference in distance between the tdx & the target & where you are.
I have Livescope & a Hook 7 at the bow, and both tdx's are mounted on the trolling motor. I don't have any use for transom mounted tdx's, as I don't fish waters where I need to know the depth when running down the lakes I fish.