Clarifying question: One ways with an opposing bike lane on the left side, or one ways with a same-direction lane on the left side? Boston (and the surrounding

Clarifying question: One ways with an opposing bike lane on the left side, or one ways with a same-direction lane on the left side? Boston (and the surrounding communities) are doing an increasingly large amount of the former. I no longer live there and never used them much, but I'd be keen to hear about the hazards of that design.