They do work for levelling up on HC by just crafting ML shards. When they fail you 'just' loose essences. And I ensure to keep the chance without booster) above 50%, unless I'm really pressed for time ...
But yeah, it feels more that a 25% booster is from x% to max 25%, where x might as well be near 0. Hence I only used them on HC for crafting any other kinds of shards to increase the chance from base (say 75%) to 100% or at least well above 50%, but it seems you still may roll a 0 ...
And the total chance is base% + d20*(0-max booster%), and unless this total is clearly at 100% you may still fail, because anything under 100% (boosted) seems to involve another dice roll, unclear to me how well balanced. Hence I ensured that the total chance after booster was 100% (or more) to avoid loosing my precious uncommon collectibles for no gain...