It's just that they didn't factor in a well-drilled Brumbies outfit, who possibly shaded the home side in a forgettable first-half and came closest to scoring when rangy flanker Scott Fardy lost the ball over the tryline.
But 70 minutes of drudgery was cancelled out by the late theatrics, with Taumalolo scoring and Cruden converting to give the Chiefs a 22-15 lead.
And still the late drama wasn't over, with Brumbies replacement lock Sam Carter given the nod by the TMO in a tryline pile-up that took an age to decide.
Lealiifano, a former Waikato player, coolly slotted the conversion from near the sideline to tie it up again. But Willison, who'd had a hitherto quiet night deputising for Richard Kahui in the midfield, came alive, getting on the end of a stellar lead-in by his pack to seal the win with a touchline tickler.
Brumbies first-five Christian Lealiifano opened the scoring in the third minute when he banged over a penalty from 40m before Chiefs first-five Aaron Cruden squared the ledger six minutes later after spoiling work from halfback Tawera Kerr-Barlow at the base of a Brumbies scrum.
Cruden nudged the Chiefs in front while Lealiifano missed a chance to level before making amends midway through the first-half when the Chiefs were pinged for offside.
The home side's best period came in the 24th minute when they used the bulk of Mahonri Schwalger and Sonny Bill Williams to bust up the right flank, although the long movement was blunted by the Brumbies infringing close to the gain line, for which they were punished by Cruden.
It was a messy first half, where the exertions of wins over the Blues and Crusaders seemed to have taken its toll.
Mind you, if the Brumbies had spent the first half any further offside then they might as well have set up a tent over the dead ball line, a ploy that took the teeth out of the Chiefs' attack.
Lealiifano put his side ahead with another kick before the Chiefs sparked into life, a Tim Nanai-Williams bust giving Cruden the chance to make it 12-all 80m downfield. The first-fives traded kicks and 15-apiece should have been where it finished.
But Taumalolo and Willison had other things on their minds.
Deserved or not, the journey continues.
Final score
Chiefs 29 (Sona Taumalolo, Jackson Willison tries; Aaron Cruden 5 pens, 2 cons)
Brumbies 22 (Sam Carter try; Christian Lealiifano 5 pens, con). Halftime: 9-9.