It was always an audacious move for Jonathan Mills – the Australian composer who stands down as director of the Edinburgh International festival this year – to devote half of the last concert of his last festival to a large-scale work of his own.

Sandakan Threnody (2004) is an oratorio for symphony orchestra, chorus and tenor soloist. The name refers to a Japanese prisoner of war camp in Borneo where many Australians, including Mills's father, were incarcerated during the second world war. Its texts include Psalm 130 as well as excerpts from Anna Akhmatova's Requiem and Randolph Stow's Outrider anthology. One wonders whether the work would have been programmed had it not been composed by the festival director. It is heavy-handed and poorly constructed; it makes very little impact with a great deal of fuss and flowery rhetoric. Some of its failings are down to questionable taste: the overly insistent piercing notes, the banal rhythmic motifs, the reliance on gimmicky percussion to mask a lack of skilful orchestration. Other problems are more fundamental, such as forcing the soloist to struggle against scoring that is far too thick.

Tenor Andrew Staples coped heroically, as did the BBC Scottish Symphony Orchestra, Edinburgh Festival Chorus and conductor Ilan Volkov. It was doubly audacious for Mills to pair his Threnody with Janáček's Glagolitic Mass, a work of staggering imagination. This is Catholic liturgy set in old Slavonic; Janáček's god is fearsome and elemental, his soundworld awesomely earthy. The original edition of the score – championed by Charles Mackerras and adopted here by Volkov – is especially visceral, and the BBCSSO gave a tremendously full‑throttle if occasionally rough performance. The chorus and soloists (Hibla Gerzmava, Claudia Huckle, Simon O'Neill, Jan Martiník) went for operatic scale and drama, fun if not always nuanced. Thomas Trotter's rip‑roaring organ solo got the biggest cheer of the night.