Generally K gaskets fail due to lack of coolant. Problem with the K - part of the initial design goal that it had to heat up in a minute or 2, is that it has a tiny amount of coolant - about 2.5-3l. Smallest leak, an its run out.
Now, the K was one of the first all alloy engines for mass produced cars. At slightly above normal combustion temps, you get a strange reaction in the alloy joins to HG.
The K also introduced a new cast technique, but wasn't really refined in the BMW era, added to which it is a very lightweight engine, there can be scope for minor distortion (not helped by the long clamping bolts).
So the engine has its flaws, that should have been refined out, but BMW were not prepared to put any R&D money into Rover beyond the ill fated 75 (which BMW knobbled to protect Series 3 sales), the R30 which becames the 1 series, and of course the mini.
But the people who worked on the engine, and those that are rebuilding, do say in most cases, something else happened that caused the higher temps, which causes the HG to fail. With the HG being a symptom.
Particulary bad are the VHP ones and the ones in MGFs, potentially due to distance between rad and engine - the engine should have been reworked for rear engined applications