Phil,
You are right, AVCS is a very usable mode for fixed wing models, especially on T/O provided one knows how to establish radio neutral. I have no clue why the old wives tales have discouraged Heading hold, AVCS, tail lock or what ever the radio manufacturers call it. There are some issues when certain models have adverse control coupling, but at lower gains these issues are managable. Each model should be approached through testing using lower gains in rate mode then AVCS, followed by an assessment at safer altitudes. I always program logic switching (2 switches in a certain position) before AVCS is enabled. For testing I use a gyro off switch position as well. On the prop planes I have finished testing, I have been able to eliminate the off position since rate mode gains are now well tested. I am now beginning to adjust gains for my jets as well. One of them is the legacy Flash. I hope there is enough season left to dial it in. That poor airplane has bad ground handling tendancies depending on power application. It was always squirrily under braking. This is a classic benefit of AVCS.

I'll test rate mode first then T/O in AVCS and see what gains it needs. Cross winds are the best conditions for the testing.