FWIW, the Hammar H20 HRU incorporates both
a) A hydrostatic release unit
b) A weak link
The attachment of the painter to the ship is via a loop of 8mm (5/16") double-braid rope
(typical breaking strength 1820kg / 4000 lbs, manufacturers quoted minimum breaking load 1630kg / 3580 lbs)
In the the event that your boat sinks and the HRU is activated (by hydrostatic pressure, typically at around 2.5m - 4m depth
(8' - 13'), then the double braid rope
is cut, but the weak link remains, so the liferaft
painter is still attached to your boat, and will not simply drift off into the distance.
The Hammar weak link typically has a break strength of 2.2kN (225kg / 490 lbs), so that when the HRU has cut the 8mm (5/16") rope, you still have to exert well over 200kg to break the liferaft
free from the boat... again, preventing the liferaft from simply drifting away.
The theory is that your liferaft has way more than 220kg of positive buoyancy, so that your boat sinking will not drag the liferaft under, but will break the weak link.
That's the theory, anyway.
For me, if you want to save yourself $150, attach your painter to the liferaft with a loop of 3mm (1/8") double braid, which will have a breaking strength of around 170kg (375 lbs) and you'll get change from $1... it will serve the same purpose.