I never tried one, but I was told they were hard on the knees.

A couple bikes in my family's fleet have Biopace rings, both having been made around the same time period. What I recall at the time was that they were greeted by a collective yawn because they weren't eccentric enough to have much of an effect.

I just measured a 48T Biopace ring that was in my spare parts bin, and the ratio of major to minor diameter is about 3 percent.

The only annoyance I can think of is that you can't use those rings with a single-speed or gearhub without a chain tensioner.

If they're wrecking your knees, don't push such long gears. Or, rotate the ring by 72 degrees on the crank to pretty much negate the effect.