One of the main problems for leggi is that he takes the ball away from right handlers and into lefti. So, for lbw, it’s really difficult for them to get any call in favour unless batsmen are dead on back foot. Conventional leg spin will pitch outside leg if it’s to hit sticks, that takes LBW out; while for left handed batsmen, often if it’s to hit sticks, pad-contact will happen outside the line - unless batsmen are not offering shot, won’t get LBW.
Now, googlie can be effective for right handlers to attack the sticks, but again unless they are on back foot, either point of contact will be outside off line or it’ll slide down the leg. For lefti, a googlie has to pitch outside leg to touch the sticks or edge, otherwise batsmen will just keep leaving.
To counter this problem, I believe first it was Tiger O’raily who started to use flippers, basically a top spinner that pitches on line and straightens, mean if it hits the pad, contact will be within line. Later Benaud mastered the art of flipper.
Still, still a top spin player can play out flippers on front foot and not playing for spin - Salim Malik was a master of that from PAK while all four Indians of 90s could read the flipper in flight, gave Warne a tough time. Also, to be effective, you have to bowl flippers/top spinners a bit faster which gave batsmen a clue and Indians used to pull Warne of his flippers, if it was remotely short.
Finally, comes the most lethal weapon by which Warni almost killed a full generation of England, PAK & SAF batsmen - the slider. It’s basically a flipper that pitches within line and straightens enough to strike pad within line. Difference is, slider is a proper spinners ball, bowled at a much slower pace, with high flight taking the ball above eye line and it drops on proper length, giving batsmen very little time in cut, even if he could identify the ball. On top of that, Warne could drift the ball on air from outside off, often catching batsmen cramped around front pad (stressing front leg too much), to pitch on middle rather than outside off (or leg for lefti), and it will straighten dead straight - basically if you are stuck on pad, only height can save you unless somehow you get an inside edge.
Of all the variations of leg spin, google looks most lethal because it turns lot and brings many other moods of dismissals; but the most difficult ball to negotiate is a perfectly flighted slider, bowling into wind so that it drops properly and drifts in air.
