Consider this: all legacy 100W commercial HF transceivers which have an in-built ATU are
reducing the output power to about 10W in tuning mode.
The reason is to protect the ATU against excessive voltages.
I doubt if any automatic tuner rated for 100W will be happy when tuned with 100W and the same
applies for the transceiver.
MFJ has a warning regarding maximum power at tune up for their auto ATU's: 100W with fold-back
and 20W without fold-back for a 200W model.
I don't know if I would be very happy to rely solely on the transceiver's final protection
circuitry (power fold-back at high SWR).
Analogue, the QDX rated for 5W output won't budged when using an automatic ATU and using 0.5W
in tuning mode.
I don't own a QDX, but my 40M QCX was saved by the current limiting of my power supply when
accidentally tuned with 4W output. With 1W output no problems are encountered.
To sum it up, it is not realistic to expect unprotected finals to survive when tuned at full