The only cited disadvantage I have seen for solo 401k's are fees to administer them. My broker has no fees, so we can cross that off the list.
Given my position to max out a solo 401k and contribute X% of profits to it, what advantage does an IRA have at all, given its contribution limit being such a fraction of the solo 401k? I am aware of SEP IRA's which also have a high contribution limit so we can fact that into this question too.
Assume only pre-tax plans, so no Roth. This is not assuming that IRAs are mutually exclusive of owning a 401k plan.