If anything, the autoformer allows you to have stable rated power on the selected ohm taps. If you have 2-ohm speakers, use the 2 ohm taps and you will have stable power at 2 ohms. Try that with an amp rated at 8 ohms which can also do 4 ohms, but not 2 ohms. You will lose power. No matter what you do, the amp and speakers need to have synergy regarding the ohm rating.
This was an extract from a review in Germany for the Mcintosh MC2301 tube amp, which were the first tube Mcintosh product to use the autoformer.
The power output may change based on which ohm taps are used. This is because the MC 2301 provides taps for 2, 4 and 8 ohms. The output stage is not interested in nominal values, it is influenced by the complex, frequency-dependent "fluctuating load of the connected speaker", the proof is in the pudding. Whatever sounds best is allowed.
Using the 4-ohm taps, the measurement laboratory from Audio.de determined a tight 353 watts with a simulated 4-ohm termination, while the power output at 8 ohms was limited to "only" 271 watts. On the 8-ohm windings of the output transformers, the MC 2301 produced 350 watts into 8 ohms and a satisfactory 154 watts into 4 ohms.
Overall, I think you have more versatility with an amp that provides more than one set of ohm taps. On the other hand, you match the speaker and amp which does have the correct ohm rating for the speaker, then you are okay as well. Where I see that as a drawback, is if the speaker is rated at 8 ohms, but dips to 4 ohms, yet your amp does not have a 4 ohm tap, nor has good 4 ohm performance with the 8 ohm tap. Then, you could run into a mismatch as the speaker dips down deep into the low ohm range.