Yeah I also find that reversing harnesses only recondition one channel, introducing delays to the reconditioned ( reversed ) channels.

These units often feature "pots" to make the adjustments, and these pots wear out easily with use. Also the crystals used drift with temperature changes, forcing frequent tweaks to the fragile pots.

Better to use a "reconditioning" device such as a JR Matchbox if you want to do this outside of the TX.

The Matchbox takes the incoming signal and sends it through it's own dual ( or quad in this case ) circuitry so that everything is in sync.
It can also be separately powered lowering the demand upon existing electronics. That's why we often see Matchboxes and the like on larger more expensive planes.

For smaller planes I prefer mechanical solutions as much as possible, then dual channels.