I'm no lawyer, and therefore may be invoking Cunningham's Law - if so, please correct me!

According to my recollection and brief searching, sworn affidavits taken under oath and making false statements in a sworn affidavit is therefore considered perjury.

However, they're not usually considered as evidence without the witness is deceased or otherwise incapable of giving evidence in court. I believe they may be used to refresh a witness' memory.

Again, I'm not a lawyer, nor am I even in the US, so exact judicial conventions etc. are beyond me, but I see them as being less serious than court testimony. Possibly serious enough to get a case to court, but if witnesses are then unwilling to testify in court I suspect the case would not get far.