In their kernel they're using a subset of 1990s C++ that gives them much less functionality than the C++ aficionados have been talking about in these comments.
All non-core components use only C APIs even though they may be in C++. So many of the examples mentioned for Linux would still have to be done the same way since they're available in blessed module APIs for Haiku.
And despite a decade's work what they have is basically a BeOS clone. Nasty shortcuts to rush BeOS to market before Be Inc. ran out of money, faithfully reproduced. That goes from big picture things like no privilege separation and no power management to little annoyances like no real hot plug (they have a hack that lets them hotplug USB devices by first loading all the drivers they might want...)