NSExpression works great on the simulator and does not work on the device when using %@ and/or stringByAppendingFormat:

evaluate.m

- (void)evaluate {
    NSExpression *expression = [NSExpression expressionWithFormat:@"FUNCTION(source.firstName, 'stringByAppendingFormat:', ' %@', source.lastName)"];
    NSDictionary *expressionObject = @{NSMigrationSourceObjectKey: @{@"firstName": @"tacos", @"lastName": @"burger"}};
    id result = [expression expressionValueWithObject:expressionObject context:nil];
    NSLog(@"%@", result);
}

//on simulator: "tacos burgers"
//on device: "tacos  %@"