Reasonably, criticism of Sahaba must be distinguished from slander.
'Slander' according to Longman Dictionary of Contemporary English is defined as:
a false spoken statement about someone, intended to damage the good
opinion that people have of that person.
Therefore, if a statement provides even an apparently factual or reasoned criticism of some Sahaba or even expresses a general scholarly point of view held by a group of Islamic scholars in regards with Sahaba, the position will not fall under the definition of 'slander' as it doesn't indicate ill slanderous intentions unless they are firmly proven wrong and the non-scholarly ill intentions behind them demonstrated.
But generally as for disagreements on merits of Sahaba you should note that they are some of the very issues that deeply divide different sects of Islam. A sahaba that is considered 'righteous' by one sect may be denounced as 'unrighteous', 'wrong' or even outright 'treacherous' by another.
Therefore members of each sect have the right to express their beliefs so long as they are referenced and backed up by their official sources. If you don't agree with them, then the best you can do is to question their opinion by providing counter arguments.