Sion W seems less stacked than newer champion/reworked shields
I just got out of a game recently and noticed for the first time that Ornn's w provides "Displacement Immunity" like a Warwick q would. I don't have a problem with it, of course our Yasuo top couldn't do much, but that's fine to have, as it would feel bad to bust out such a short range skill and have it do zero damage due to cc. As for Urgot, in his recent rework his w was changed in a way for him to stack fever very fast for a tank, and slows enemies to about his own to keep them in range.
As for Sion, his W has stood the test of time since his rework early season 5, and while I know it has the health stacking passive(which really looks like im going to try out with the new health stacking rune in pre-season 8) it has a significantly longer cooldown, and its only combat bonus outside of the shield is that it can blow up, assuming it isn't destroyed during the trade you prep it for. Even Nautilus gets immediate value for trading from his shield through a mini sunfire built into it, and its bigger and than Sion's during the lane phase when tank trades can actually matter. Sion's shield seems to be more useful as an extra waveclear tool, and maybe 300-500 bonus health in late game teamfights, but 300-500 isn't much considering the meta is crit adc with tank frontlines.
Of course I shouldn't base his shield effectiveness on meta, but considering that hes supposed to be a giant full tank with reliable cc, the W seems like a boring skill that you only rank up for the passive. I honestly have no idea what I would change, but it seems to have very little use, especially if its gone before the 2 seconds are up to deal your 100 magic damage(which seems to be the most common damage value i get back after popping a level 1 W on a lane opponent).