What bugs me about force-field belts, aside from the lack of a safe failure mode, is: Where the hell are the air tanks? A skintight force field wouldn't give you a lot to breathe. I guess there could be a limited supply of compressed air inside the "power pack" bulge in the rear, but not a lot. (For that matter, the "Tholian Web" spacesuits have the same inexplicable lack of any apparent air supply.)
I liked them, and they'd have been easy enough to do on live tv, as well. Just draw the glow the same way phaser beams were drawn in.
That would've been prohibitively expensive to do continuously for however many minutes the characters would be in the fields. Also, it would've had to be hand-rotoscoped to match the characters' movements, which would've been kind of scrawly and jerky-looking. This wasn't a problem in TAS since the characters hardly moved, but live action would be another matter.
But then, live-action force fields are usually invisible anyway. Most likely what they would've done was the same thing Filmation did years later in the live-action
Jason of Star Command (and maybe its predecessor
Space Academy too -- I forget). They had little boxes on their waists that supposedly cloaked them in invisible force fields to operate in space. You couldn't see the fields, but you knew they were active because the characters were shown turning them on and there was dialogue to that effect. Maybe there would've been a bit of animation to show the force field coming on and then turning invisible, like they usually did with force fields in TNG and after, but it wouldn't have been continuously visible.
Heck, the regular SF uniform could easily be laced with nanotechnology (which SF did have before encountering the Borg) and metamaterials that can easily serve the same function as a tricorder, not to mention administer medical aid and potentially fix other issues for the wearer... but of course, again, the writers were pretty limited in their thinking.
There's a neat, overlooked bit of clothing futurism in, of all places, "Spock's Brain," where the landing party beams down to a frigid world and Kirk tells them to set "suit temperatures to 72," which they do by manipulating some unseen controls on their belts. Implicitly, their uniforms had built-in heating mechanisms. Of course it was just a way to save money on costumes, but it was a nifty idea. Unfortunately, then TWOK went and introduced heavy parkas for landing parties, and Trek went back to the assumption that the uniforms were just inert cloth.