If the aliens can give Simon Pegg the power to do anything...
then I presume they also have the power to do absolutely anything. So why can't they fix the machine that goes wrong quicker (why don't they make a machine that can't go wrong in the first place?) and ultimately, why are they bothering to do anything they do in the movie when they could just make the whole universe exactly how they want it and not have to waste time bothering with what they view as inferior lower life-forms.
The eternal problems with wish-fulfillment movies (and time travel movies, and some other types of story based on an impossible premise) are that there are no stakes because the protagonists can always reverse or re-do anything that doesn't work, so the story kind of invalidates itself from the outset. Any kind of plot is intrinsically pointless.
So yes feel free to call me a kill-joy who is over-analysing something which is just meant to be a bit of fun etc etc but it deserves criticism because it was a pretty bad movie in all departments and a massive waste of potential given all the talent involved. Great shame. Writer/director Terry Jones is a lovely guy and obviously has a great legacy in the history of comedy, but he made a mistake on this one and the millions that were no doubt spent on it I think. Erik The Viking is much better.