People proclaim this with such confidence, especially in the UK before the Brexit years I must say, and yet there are so many arguments against it.
First, accepting this goes against a core tenet of the single market, since the single market for vacuum cleaners is now split into 27 little markets. We thus need 27 regulatory bodies to each investigate, and then make a political decision, on the maximum power setting of domestic vacuum cleaners. Then, producers in China will have to make (up to) 27 different models of vacuum cleaners. This is certainly not going to be cheaper, completely destroys the famous Brussels effect [0], all for some token amount of "sovereignty".
Nations states can sovereignly decide to delegate this process of rule making to Brussels, and domestic vacuum cleaners are a perfectly mundane "case in point" to showcase the benefits of this process. The EU is far from a perfect union, but throwing these frankly naive arguments around will not help it much.