It is not uncommon for refusal letters to contain some variation of the following statement:
I find that letters or affidavits made by the applicant, who has a personal interest in this matter, require a degree of corroboration in order to contain much weight.
As the Federal Court of Appeal held in Maldonado v Canada (Minister of Employment and Immigration),  2 FC 302 at para 5 (FCA), this is inappropriate. There, the Court noted that:
When an applicant swears to the truth of certain allegations, this creates a presumption that those allegations are true unless there be reason to doubt their truthfulness. On this record, I am unable to discover valid reasons for the Board doubting the truth of the applicant’s allegations above referred to.
In Hilo v Canada (Minister of Citizenship and Immigration),  FCJ No 228 (FCA), the Court further noted that:
The appellant was the only witness who gave oral testimony before the Board. His evidence was uncontradicted. The only comments as to his credibility are contained in the short passage quoted supra. That passage is troublesome because of its ambiguity. It does not amount to an outright rejection of the appellant’s evidence but it appears to cast a nebulous cloud over its reliability. In my view, the Board was under a duty to give its reasons for casting doubt upon the appellant’s credibility in clear and unmistakable terms. The Board’s credibility assessment quoted supra is defective because it is couched in vague and general terms. The Board concluded that the appellant’s evidence lacked detail and was sometimes inconsistent. Surely particulars of the lack of detail and of the inconsistencies should have been provided. Likewise particulars of his inability to answer questions should have been made available.
The jurisprudence is similar in cases involving refused declarations.