His choice of stainless steel is panning out well here - I doubt if aluminum or composite body structure would hold up as well to the "grab" forces from even minor misalignment. A composite structure would likely be entirely compromised by a big scrape.

It would be interesting to see a test where the landing speeds were deliberately too high - how much deceleration can the arms handle safely?

I think the chopstick mechanism is probably the best possible catch mechanism for such a tall object. The booster will be suspended from the top, which means the booster isn't subject to tipover as it would be if landing legs were involved. We've already seen this many times in the Falcon 9 booster series.

I can't see chopsticks ever working from a droneship, though - too much induced rotation for chopsticks to compensate.

As an alternative to chopsticks, a catch 'sleeve' might be possible, though it would magnify alignment errors considerably.