Many of these one-participant plans have < $250k of assets and don't need to file 5500-EZ while operating. Still, a Final 5500-EZ is required when the plan closes, and this is often missed. I've never heard of consequences from not filing a Final 5500-EZ but maybe a future IRS/DOL will datamine that filing miss to generate revenue.
I will say, while most advisors will just use the plan documents provided by a custodian (Altruist/Charles Schwab) without even reading them, there is a growing group, especially in XYPN, that will typically bring in providers like mysolo401k.
But I mostly see advisors using the custodian's plan docs, and the advisor takes on a pseudo admin role without having the knowledge to step into those shoes.