Motorola roundie

[old_tv_nut]

I'm surprised that only one adjustment was needed.

With the fixed matrix resistors, you can adjust I/Q relative phase and the COLOR control to compensate for errors in one color matrix, say red. Then you still need I and Q gain adjustments in both green and blue to get them perfect, so that's four additional adjustments total.
If you insist on the I and Q waveforms being exactly correct, then you need I/Q ratio adjustments for errors in all three matrices, plus luma vs chroma gain adjustment in two of them, so that's five additional adjustments.
As you say, these adjustments can be avoided by the use of precision resistors in the matrices.