> Boarding times though?

Often true. OTOH, the reduced footprint (vs. either 1 articulated or 2 normal busses) can speeds things up, especially on cramped and congested city streets. And if you're getting near gridlock-level congestion, the length of the articulated busses can be like dumping gravel into your system's gearbox.

True. While I love our articulated buses in Kraków, if one of these stops mid-turn on an intersection, it blocks 3 out of 4 movement directions.

These issues are solved with dedicated bus lanes, which are desirable even for double deckers.