There was a media flurry when Michael Flynn announced that he would not testify before Congress, citing his Fifth Amendment rights. Democrats denounced it while Republicans said it was no big deal. Of course each party took the opposite position when associates of Hillary Clinton took the Fifth. Hypocrisy is alive and well in Washington.
I have no idea whether or not Flynn committed a crime. That’s not the point. Congress has refused to grant him immunity. That means everything he says can be used against him. Any lawyer would advise him not to testify. Even if he was granted immunity, there are still problems. Oliver North was given immunity. While prosecutors couldn’t use his testimony, it still affected their ability to prosecute. He was convicted but it was overturned. The court ruled that his testimony affected the testimony of others at the trial.
Either there should be a congressional or a criminal investigation. Trying to both at the same time reduces the likelihood of either being successful.