I try to tell the designer/playtest coordinator stuff like:
* These reinforcements need more time, because...
* I didn't like this about the scenario...
* Here's what would happen if I did this...
* The scenario will have a higher replayability if you did this...
* A SSR like this would fit here...
* This and that was really great...
* Etc...
Playtesting isn't always about balance. Designers want to hear about other aspects of their scenarios. They ALWAYS want to put out the best possible design and any suggestions or ideas that you can give them will be taken into consideration. So, it doesn't matter if you make stupid mistakes, they just want feedback.