There's two separate points there...using a flex nib, and actually doing full-out Spencerian. :) I've seen Rob Morrison doing it, but that was a LONG time ago at the LA Pen show. (We're talking 20-odd years ago.) I think the hand/wrist position is more open than one commonly sees with regular nibs.
And he may have also been using the index-middle finger grip, which is more neutral. I played with that but never got it down pat.
You may well be right about unbalanced stress...and stress in general *is* a fundamental issue with flex. The more flex, the more susceptible to ongoing metal fatigue, ultimately leading to a sprung nib or a cracked tine. Or sometimes a crack at the base of the nib.
In some ways this is a better example...still showing a LOT!!!!! of line width variation, far more than most will ever want. Also copied from the web site I cited earlier:
The hairlines are very fine; the emphatic downstrokes are, what, 5-6 times wider. If you wanted to try this.....and this is a lot closer to 'typical' use of flex, with consistent heavy downstrokes and lighter cross-strokes...is the sheer ink flow you need. The J and the C...the feed has to keep up. Narrow-shouldered cartridges (the vast majority) very rarely can keep up. Note that this holds for very broad nibs too, sometimes...3x broad or some big, fat stubs.
I wouldn't recommend a flex nib for a newbie, or as one's only FP. A soft nib or a semi-flex nib....no, they're not quite the same...is easier to handle while still allowing expressiveness to enter into your strokes. If you want a flex nib, great, but be prepared to practice with it.