Of course you can buy frozen roast potatoes and yes, you can tell. Sorry but they aren't as nice as fresh. M&S ready prepared (fresh) are a good compromise though. And if no-one likes sprouts, have other vegetables instead. Last time I checked, there were several different types of vegetables for sale in the supermarket 
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However, you are of course not a failure if you don't produce the 'perfect' Christmas Dinner. Perfect to who, anyway? It is beyond rudeness to be anything other than grateful and polite about an invite to any meal or it's contents/quality.
But you should really stop putting pressure on yourself and instead, simply stop giving a damn about what other people think or say. If they complain, it says more about them, than you.
You are not a failure for living up to their expectations, they are a rude sexist old idiot for not appreciating the effort you have gone to. One of DPs uncles once made a 'joking' comment about my poor housekeeping skills because we had cobwebs and I handed him the broom and suggested that he may wish to deal with them, or else I could find something else to do with the broom.
IME the people who complain have often never cooked anything more than toast or other very simple things. Anyone who has cooked day in day out for a family will probably be much more grateful they haven't had to cook for once.
There is a piece in The Times today about how Victoria Coren (I think) says that she now enjoys Christmas so much more now that she approaches it like a man and does very little and just takes it how it comes.
Sorry to say that sometimes it is the women putting the pressure on themselves expecting everything to be 'perfect' while the men in general don't care whether there are 3 different types of meat, bacon in the sprouts, or how the table is dressed. So don't martyr yourself and just do what you want and can manage .