The Apostle Paul stated that if it were possible, he would suffer to bring about the redemption of Israel (Romans 9-10). We can clearly see here a complete contrast between his inadequate work as a moral substitute and the complete sufficiency of Christ's work. Thus, it makes no sense to say that a person can somehow add (i.e. their prayers and good works) to something (i.e. Christ's atonement) which already has infinite worth.
If the merits of Jesus Christ are infinitely valuable and inexhaustible, then it should atone for both sin and its guilt. This treasury of merit should cover both the temporary and eternal consequences of sin. Yet, the Roman Catholic Church requires its followers to make amends for the temporal punishments of sin through good works and purgatory. The benefits of the treasury of merit are not extended to eternal punishment. This seems inconsistent, given the treasury is spoken of so highly but it cannot cover the guilt of our sin. We might as well say that the treasury of merit cannot fully set one free from sin.
The treasury of merit implies that the merit of Jesus Christ is insufficient because the merits of Mary and saints are also deemed sufficient. The Roman Catholic hierarchy would certainly dispute the implications of its theology, but for what other reason would one still need the merit of another if Christ's is not already sufficient? Nobody is righteous enough to accumulate merit for themselves and other people (Romans 3:9-23). So, this treasury of merit teaching is both absurd and unscriptural.