I think that allowing shields to completely and simply block Slavers’ hooks and Assassins’ pounces would be a too strong of an ability, simplifying and trivialising those specials pretty much completely, so it would need a limitation (beyond just “need to be facing the enemy”, as that’s pretty much the default state anyway). The first thing that comes to mind is needing to use the Parry timing, even if that Trait isn’t on the weapon. It would still be more reliable than pushing (or simply hitting an enemy) and very much learnable, but would require still some skill to pull off instead of just standing behind a barrier.
While I said I’d prefer more options for blocking to increased mobility, I don’t want the shields to turn into a full-on mobile cover one can stand behind all the time. That’d just break the pacing, and would be incredibly boring to boot. Requiring timing to be used for blocking some of the more unusual stuff could accomplish making a wider variety of attacks blockable (still, only with shields) but wouldn’t particularly reward a static style.