The court previously provided those guardrails. With Chevron deference, the agencies could clear up ambiguities, but the court could reel them back if they were found to be unreasonable. Now, there is no check-and-balance; the entire power resides in the court. In the words of Justice Kagan, that is "judicial hubris."
If the court makes an unreasonable interpretation, the only mechanism to rectify it is for Congress to be explicit. This has multiple problems: first, getting anything through Congress is becoming increasingly difficult. Secondly, the Court already admitted that laws will always have ambiguities because Congress doesn't always have the expertise to be that explicit. From that standpoint, the Court has claimed a power while acknowledging there is very little in terms of a check on that power.
If the court makes an unreasonable interpretation, the only mechanism to rectify it is for Congress to be explicit. This has multiple problems: first, getting anything through Congress is becoming increasingly difficult. Secondly, the Court already admitted that laws will always have ambiguities because Congress doesn't always have the expertise to be that explicit. From that standpoint, the Court has claimed a power while acknowledging there is very little in terms of a check on that power.