Faust, Lowland Hall, Ingliston, EdinburghDiaspora, Playhouse, EdinburghOptimism, Royal Lyceum, Edinburgh

I've been to hell and back this past week. Like Mephistopheles, the Edinburgh International Festival promises the earth: the artistic wonders of the world.

Yeah, well. Prepare to nosedive. Fool that I was, I set off with an expectant spring in my step to see the veteran director Silviu Purcarete's adaptation of Goethe's Faust, presented out of town, near Ingliston.

Disembarking from the airport shuttle bus at an ominously godforsaken roundabout, I gaily followed a flock of theatre fans into a sort of

prefab hangar. Inside, Lowland Hall is an awesomely vast, dark vault where Purcarete's epic production – to give it its due – does offer some visually bewitching moments.

A ghoulish swarm of over 100 actors (from Romania's Radu Stanca National Theatre of Sibiu and its affiliated drama school) erupt from under the floorboards of Faust's study.

Storm clouds scud past frosted windows. Ofelia Popii's gurgling, androgynous Mephistopheles squats on a desk, with a bulging codpiece and her bare torso smeared crimson. Her fellow fiends, in long white coats and surgical masks, seem legion as they jiggle and writhe. However, that is one of Purcarete's crowd-choreography trademarks rehashed.

Ilie Gheorghe's Faust is, manifestly, a very bad doctor. A fat slug of a man – like a macabre clown with Grand Guignol make-up – he ritualistically injects a zombified schoolgirl with a lethal concoction. He then moves on to paedophilia, encouraged by Mephistopheles, who often operates like his evil id. That's the most disturbing aspect of this production, with the duo engaging in a shadowy sex scene with a Gretchen who doesn't look a day over 14.

The principal problem is that Gheorghe never inspires any sympathy. It's hard to imagine that he's been toiling

over tomes for years, ascetically starved of life's pleasures. He's just torpid.

Purcarete confusingly condenses Goethe's sprawling saga. Mephistopheles keeps morphing into barely introduced, secondary characters and, with lots of lip-synching, Lord knows who's talking to whom half the time.

Purcarete conceives all Faust's adventures as a theatrical illusion starring the devil in various guises, but the effect is baffling, especially coupled with English surtitles that are impossible to follow.

When the show turns into a huge promenade for Walpurgis Night, it's meant to be spectacular, with a blaring rock band and fireworks, witches suspended from forklift trucks, and (erm, why?)18th-century aristos astride a rhino.

Actually, alas, this is all a huge damp squib. The acting isn't great. The choreography is rough around the edges. And I couldn't see anything, through the crowd, of Gretchen's key scene in prison, prior to her salvation.

Such shortcomings, nevertheless, pale by comparison with the excruciating ineptitude of Diaspora. This multimedia fiasco from Singapore ought to have been fascinating and poignant. It collates personal accounts of xenophobia and incomplete assimilation from Chinese émigrés and their descendants – from Indonesia in the Sixties to Scotland today.

Yet director Ong Keng Sen's production merely looks superficially slick, with wraparound video screens and silhouetted actors on a walkway above a twangling Chinese orchestra. Ridiculously random, unintegrated images flash up: footage of a beauty competition, a suburban swimming pool, a witless Bollywood spoof. Meanwhile, one silhouette jigs and another tiptoes around like a gingerly goose-stepping burglar. The testimonies are so banal that all pathos is eliminated.

I have to say the interval proved thrilling, when a mouse arrived and sat in my partner's vacated seat. But even the rodent didn't stay for the second half. I staggered away at the end, cursing director Jonathan Mills's programming, and howling, "This is your pick of the whole planet?"

The following night, a demented punter started heckling about 15 minutes into Optimism. This is an avant-garde reworking of Voltaire's Candide, with pop songs and yodelling, by the company Malthouse Melbourne. Michael Kantor's production is quite tiresome, with feeble clowning in Pierrot outfits and mock-philosophical wittering from Barry Otto's doddery Pangloss and gawky Frank Woodley as the globetrotting ingénu, Candide. There's no biting satire or real sense of the horrors of war, colonial exploitation or natural disasters.

Still, Optimism isn't a complete flop. It offers an ecological message about the need to "cultiver notre jardin". As a Perrier-winning comedian, Woodley manages to improvise, rather charmingly, around any heckles. And the whole show perks up when he's joined by the splendidly dry David Woods (from Ridiculusmus), playing the dour sidekick, Martin.

Ensconced in jumbo-jet seats, endlessly taking off and landing, they silently slump and sit up again, under spinning chrome fans. An air hostess softly croons, "It's a wonderful, wonderful life", her arms rising and falling in a hauntingly weary semaphore of safety instructions.

It certainly wasn't the best of all possible worlds last week but, hey, all these shows have been and gone. The International Festival continues with tonight's premiere of Rona Munro's Scottish history play, The Last Witch, and things can only get better, right?