You can look up for yourself "Fincen unconstitutional". The gist of the anti arguments appear to be that the feds are inserting themselves into gathering data on matters that are totally internal to states, namely the granting of business entity certification. The formation of a corporation is supposed to be entirely a state matter. The feds (IRS) get involved in that when a corp or other entity makes revenue and starts paying payroll, they must withhold and file and document federal taxes that are due for that activity.

The IRS has to know what entity you hold for taxation purposes (which itself may not be constitutional but we take for granted.) Law enforcement does not necessarily need to know that ABC owns XYZ LLC.

I think this also plugs any gap of entities with unnamed principals, IE, the notorious Delaware and Nevada companies.

I feel the same as you, it's a very indistinct boundary between fed and state that I don't have the legal understanding to grasp why it is not constitutional.

Neither is the fed bank, neither is Obamacare, neither is relinquishing war powers to the president instead of keeping it in congress.

The short answer is "well, whatever, here we are."

The purview of this law is probably whatever you can do with a corporation that is improper. IE, taking too much profit as pre-FICA dividends and paying yourself a meager fully taxed salary. Before that would amount to an IRS action. Now probably in addition it's a ***FINCEN VIOLATION***.