rr> It is a not uncommon phrase in the commentaries with regard to
rr> anapanasati. E.g:
rr> Patisambhidhimagga -atthakatha anapanassatikatha and in the
rr> Commentary to vinaya
rr> Parajika khanda attakatha

rr> It seems that anapanasati is singled out as being the most profound
rr> object of samatha- all Buddhas develop it- but also the most difficult.

Please note that such opinion is expressed only in the Atthakatha.
In other places we read:

Some monks developed meditation through other objects, but failed to
achieve the effects as wanted. In this case the Buddha advised them
to cultivate the meditation of mindfulness on breathing instead.

The meditation of mindfulness on breathing is therefore one which
has been widely practised from the past to the present. The
important point is that, it is the meditation which can be practised
by one of any temperament.

To those of deluded temperament whose minds are absent, and who cannot
be peaceful like those of speculative temperament, the Buddha gave the
mindfulness on breathing in and out.

http://www.thai.net/winai/English/DHAMMA.htm

which gives the impression of broad applicability of anapanassati.

Kind regards,
Dimitry