The FISA court is not an article III court. Its a congressional check on the executive's exercise of discretion. It limits executive discretion that would otherwise be unbounded. So the fact that its secret isn't some big scary thing: it doesn't have the power to make decisions that require an article III court anyway. It makes decisions that would otherwise be made behind closed doors in the executive.
The danger of the FISA court is if it starts impinging on the domain of article III courts. Granting warrants that would otherwise require an article III warrant, etc. The fact that its real article III judges could lend an air of legitimacy to that process that it shouldn't have.
The danger of the FISA court is if it starts impinging on the domain of article III courts. Granting warrants that would otherwise require an article III warrant, etc. The fact that its real article III judges could lend an air of legitimacy to that process that it shouldn't have.