Ceiling roses, have the cable entry, offset to the side of the base - which means if the cables come down the side of a joist, the rose can be fixed with both screws into the joist. On the rare occasions, when I'm involved with ceiling roses, I insist on aiming for the above, or I might compromise with one screw plus one into PB, rather than make a mess of the ceiling, or add some timber above the ceiling to screw into. I'm not too bothered about smoke detectors, they are very light-weight, so just screws into the PB.
The worst fixing I ever made in this house, was the through the wall bathroom extractor fan. It went in an existing through the wall vent, from when the house was built, and there was nothing behind it the fix to, just modern plaster, from when the place was replastered. Rather than make a mess, cutting plater out, and bricking around the hole, I simply glued the fan case in place, with sticks like ****.
The worst fixing I ever made in this house, was the through the wall bathroom extractor fan. It went in an existing through the wall vent, from when the house was built, and there was nothing behind it the fix to, just modern plaster, from when the place was replastered. Rather than make a mess, cutting plater out, and bricking around the hole, I simply glued the fan case in place, with sticks like ****.