Tautology: Unnecessary repetition, usually in close proximity, of the same word, phrase, idea, argument, etc. Now typically: the saying of the same thing twice in different words (e.g. ‘they arrived one after the other in succession’), generally considered to be a fault of style. - OED
"As opposed to the above ideas, opponents say that ..." opposed and opponents sound awkward here because they make a tautology.
"opponents of the above ideas say that..." or maybe "not everyone agrees with the above, opponents say that..." would both work and would avoid the problem.
And there is nothing wrong with "It is agreed that" as long as you can substantiate the statement - it may be worth providing a reference to support your assertion.