  The detection of a wide range of substructures such as rings, cavities and
spirals has become a common outcome of high spatial resolution imaging of
protoplanetary disks, both in the near-infrared scattered light and in the
thermal millimetre continuum emission. The most frequent interpretation of
their origin is the presence of planetary-mass companions perturbing the gas
and dust distribution in the disk (perturbers), but so far the only bona-fide
detection has been the two giant planets around PDS 70. Here, we collect a
sample of 15 protoplanetary disks showing substructures in SPHERE scattered
light images and present a homogeneous derivation of planet detection limits in
these systems. We also estimate the mass of these perturbers through a Hill
radius prescription and a comparison to ALMA data. Assuming that one single
planet carves each substructure in scattered light, we find that more massive
perturbers are needed to create gaps within cavities than rings, and that we
might be close to a detection in the cavities of RX J1604, RX J1615, Sz Cha, HD
135344B and HD 34282. We reach typical mass limits in these cavities of 3-10
Mjup. For planets in the gaps between rings, we find that the detection limits
of SPHERE are about an order of magnitude away in mass, and that the gaps of
PDS 66 and HD 97048 seem to be the most promising structures for planet
searches. The proposed presence of massive planets causing spiral features in
HD 135344B and HD 36112 are also within SPHERE's reach assuming hot-start
models.These results suggest that current detection limits are able to detect
hot-start planets in cavities, under the assumption that they are formed by a
single perturber located at the centre of the cavity. More realistic planet
mass constraints would help to clarify whether this is actually the case, which
might point to perturbers not being the only way of creating substructures.
