The basic problem is the FISA itself. The FISA came into being in 1977 following revelations in 1976 by the Senate's Church committee investigating illegal surveillance of US citizens during the Nixon administration. As such, FISA was intended to define mechanisms to protect citizens from unconstitutional surveillance efforts. However, the actual effect of FISA has been to establish a secretly-functioning mechanism to do the very thing it was designed to prevent: the unconstitutional electronic surveillance and secret-court investigation of US citizens.
Article III of the Constitution limits Federal judicial power to resolving specific disputes or controversies between adverse parties for which the dispute can be resolved by a final, specific judicial decree. None of these exist with respect to secret FISA court actions, since no "disputes" between parties are identified. The only party appearing before the FIAS court is the government, and all the proceedings involve issues incapable of final judicial resolution between parties... hence no appeal path exists within the FISA courts involving affected citizens.
Moreover, Amendment IV of the Constitution (freedom from warrantless search) is violated by FISA in multiple ways, most of which have long been argued with regard to FISA as it's now applied, and which are fairly well-known.
Finally, having a secret court system violates due process clauses enshrined in the Constitution's 5th and 14th Amendments. Due process is a set of foundational principles allowing citizens to contest in open court any government action against them... that, in turn, requires notification to the citizen of governmental action, a fair trial, and unbiased judges or juries. FISA courts operate in secret, without notifying affected citizens of their actions, without citizen representation at trial, and without opportunity to challenge bias in the judges.
As it currently stands, the FISA threatens to become (and, in some cases, is) a Star Chamber secret court akin to those that oppressed English citizenry during the 1400-1600's. Prevention of these very abuses underlies much of subsequent English law and the US Constitution as a whole. Accordingly, the FISA needs to be either grossly overhauled or entirely thrown out.