I will venture a comment on daVinci tandems. We strongly considered buying one for our first tandem but decided against it for the very reason some love it. The design's goal is to allow some break in connection when coasting between the two team members. While this can be a great feature for some we were interested in the team aspect of tandems and have found the benefits of a direct connection well worth the challenges. I think of tandem riding as truly dancing with a partner on the pedals. While it would be easier to dance if one partner could pause every once in a while it still would not be the same as dancing until the music stops. Purely a subjective feeling.
Objectively the costs are some small amount of weight and complexity and some loss in drive efficiency. Even at 98-99% chain efficiency the added drive chain will cost you 1-2% of your team's power. That may not sound like much but if you are interested in speed think about losing those watts. Of course some don't care about a loss in power or a little weight and would much rather have the ability to coast any time they want.
All that being said, I have purchased cranks and other items from daVinci and must say they are a great bunch that have great products.