Neither are steadfast. If we relied on either the Legislative or Judicial branch to take action granting cohesive resolve for Confederates post-Civil War - in lieu of Lincoln's 12/8/63 amnesties as a power vested to him through executive pardons, the Union would have collapsed. Outside of amnesty from Lincoln, it took the next acting branch - the Legislative - 7 years after Robert E. Lee's 1865 surrender for their actions regarding the Civil War to be seen (1872 Amnesty Act).
And, in the systematic event that a law is passed that is grotesque - from the legislative, or in the individual event that a miscarriage of law is applied to an individual case - from the judicial; we need a quick check and balance for either scenario - and the Executive branch is (typically, and on average) the fastest-acting branch of the 3. Lest, one bad-faith branch can reliably depend on its complementary power to be too slow to act (which is happening now, in many ways).
As a result, the executive needs to add tension for either event, and just "Legislative <-> Judicial" having checks against one another in relation to laws, and the judicial proceedings concerning the laws, is not enough.
