A departure from the landing config is highly unsafe.
With all the instructor I had, touch and gos were always performed like this:
- Bring aircraft in the landing configuration (L/D flaps, carb heat on)
- Land the aircraft
- With idle power, bring aircraft back to centerline, and bring aircraft in the take-off configuration (T/O flaps, carb heat off)
- Once stable on the centerline and in the take-off config
- Full power
- Rotate at Vr
Obviously, if you forget to retract flaps or select carb heat off, you're in for a lot of surprise.
The most difficult manoeuvre is actually the go-around, where you've got to do this in the air without losing altitude.