Well, I can reveal that I playtested a couple of scenarios using the IIFT (with CTC) with the designer which have later been published by respected companies and that have received praise by people who'd never touch anything but the IFT. I make no claim that these have exclusively been playtested using the IIFT.
To spare the IFT-purists the mental pain of having been "betrayed", I won't tell you which ones...
I think that it depends on a number of factors if a scenario turns out to be balanced or not.
In some scenarios, using the IFT or IIFT might play more of a role, in others hardly any.
Some scenarios are balanced for very experienced players but not so for less experienced ones and vice versa.
The list could go on.
We also see, that many scenarios, of which we know they have been thoroughly playtested, still are not reported as 'balanced' in the long run after many playings on ROAR and/or the ASL Scenario Archive.
Overall, I believe, archieving balance is more an art than a science on one hand and also depends on the players involved on the other.
von Marwitz