  The possibility that rotating black holes could be natural particle
accelerators has been subject of intense debate. While it appears that for
extremal Kerr black holes arbitrarily high center of mass energies could be
achieved, several works pointed out that both theoretical as well as
astrophysical arguments would severely dampen the attainable energies. In this
work we study particle collisions near Kerr black holes, by reviewing and
extending the so far proposed scenarios. Most noticeably, we shall focus on the
recently advanced target particle scenarios which were claimed to reach
arbitrarily high energies even for Schwarzschild black holes. By implementing
the hoop conjecture we show that these scenarios involving near-horizon target
particles are in principle able to attain, sub-Planckian, but still ultra-high
center of mass energies of the order of $10^{23}-10^{25}$ eV even for
non-extremal Kerr black holes. Furthermore, analysing the properties of
particles produced in such collisions, we find that photons can escape to
infinity. However, their energy is only of the order of the energy of the
colliding particles and hence relatively low, which is the same conclusion
previously reached in the literature about the original Ba\~nados--Silk--West
process. This finding points towards a general limitation of collisional
Penrose processes, at least for what concerns their primary products.
