The ECJ has been the court of last option in the UK since we joined the EU )I think). On leaving the vast majority of its rulings were adopted into UK law. There was no criticism of its rulings until now. It seems Johnson seems to think that trade deals with Europe should be supervised by UK courts which he does his best to manipulate. He has been quoted in the past as saying ‘fuck business’ and I suspect that he is not troubled by the potential return to the ‘troubles’ as he seems to be immune from the consequences of his cock ups.
Basically there never has been, and is ever likely to be any major disagreement on the rulings of the ECJ regarding trade deals unless our government decides to repeal aspects of environmental and labour rules that at present are fairly uniform across the forma 28 EU countries.
The EU also has a vague opportunity to influence the coal dependant countries of Eastern Europe – whilst Johnson may as well piss his thoughts up the wall.