Typically owner-only plans make up 10-20% of the TPA book, with multiple owner entities comprising about 20% of them, usually partnerships or a successors arrangement with a younger minority arrangement. Almost all will have a financial advisor involved; but usually SDBA accounts with each owner engaging his own advisor. Biggest issue is lack of formal Plan Document, too many brokerages make it seem like their account application will cover the doc requirements in these arrangements.
As a TPA, my focus is on providing services centered around ERISA and related IRS/DOL regulations; state law is only considered specific to particular divorce and maybe probate benefit situations when invoked. Even where ERISA may not apply, I'll lean on those precepts for guidance and shape my communication accordingly.