The 'down side' is that it drags out a sham trial where Trump's guilt is already presumed - AND it is keeping Trump off of the campaign trail.
However, I seriously doubt that putting Costello in the witness box is the 'big question' to Judge Merchan. He has denied witnesses with exculpatory evidence already.
Judge Merchan has obviously committed unethical bench practices. If the case goes to appeal there would probably be an investigation by a judicial ethics panel plus the appeal court's decision based on the merits of the defenses' charges of misconduct.
The New York Dem Party may be insane enough to drive this into federal court but I tend to doubt it. Notice how Judge Merchan reversed his decision to deny Trump's attendance at his son's high school graduation. Obviously someone was sensitive about the tawdry appearance of the judge's autocratic oppression - in a case that is destined to eventually end up in a trash heap - with Merchan judged a biased unethical judge. 🤷