It is better to land and pull off the runway, changeover and takeoff again not just from a simplicity angle but also because the new pilot does a complete flight rather than being dropped in it!
Pace, depends on what type of training you are conducting. I agree that for ab initio it may not be a good idea.
However for advance students on Instrument Rating training I see no reason why not. We used to take 3 students in the Twin Comanche/Seneca - first student would fly airways to XXX do some approaches to touch and go/go around, 2nd student does airborne change to do approaches at XXX, 3rd student does airborne change to do approaches at XXX and then airways back to base.
Given that base at that time had no ILS this meant all 3 students got to do at least one ILS each (usually more) and also a non precision approach.
The main challenge then (and I suspect now) is that if you landed to do a crew change it was challenging getting back into the system again to restart training and often resulted in delay at the holding point.
For advanced students who, within the airline context, have to get used to airborne changes, this was good training to prepare them for the future. Also they had the benefit of watching their fellow trainees perform and also learn from the debriefs etc.
Of course airborne changes have to be briefed and correctly managed but if the left hand seat was moved fully rearward and the right hand seat moved forward a tad there was never any problem changing seats.
I suppose if you have never done it before it's like going to the moon (you have to solve two problems:- how to get there and how to get back) but airborne changes in this context have been performed quite safely many times.
As a postscript there is always a risk somebody might fall over the controls when exiting or entering a pilot's seat. Another reason for covering this in training.