Ruaraidh

Looking at the shrapnel a few posts up I'd imagine that the valve floated at high speeds, the hyd lifter pumped up to take up the new "clearance" caused by the float and the next time the cam opened it pushed the spring right into coil bound breaking everything and most likely causing the valve to piston contact which has then shredded the whole piston/bore and head.

The main danger of hyd lifters (roller or otherwise) is that if for any reason, valvetrain control is lost, then the lifter will pump up and then next time the valve opens damage will occur. If it hasn't pumped up a lot, then you might get a bent pushrod or rocker but if it's pumped out a lot (and it will incrementally pump up for each valve event that floats) then eventually the spring will go coil bound and the valvetrain will be destroyed at that point.

A solid lifter (flat or roller), with a cam that has been designed for it ie it has lash ramps to take up the clearance, will generally be lighter, stiffer and more tolerant of any valvetrain loss of control than will its hydraulic counterpart. The price you pay is the maintenance requirement.

I'd imagine that whatever the chosen adjustment mechanism, you'd want to use a roller lifter as you can get a) more opening flank velocity and b) lower stresses than the equivalent flat lifter solution.

Hope that helps.