Instability of gem dihydroxy compounds
The instability of gem-diols is due the fact that hydogen bonding between the 2 OH- groups increases the repulsion causing a strain on the C-O-H bonds.
In case of compounds of boron, a greater bond angle (due to sp2 hybridization of boron) prevents intramolecular hydrogen bonding.
Compounds of sulphur, phosphorous always contain a double bonded oxygen along with the OH- groups on the central atom. This oxygen atom forms hydrogen bonds with OH- groups preventing hydrogen bonding between the OH- groups, thus preventing any strain.