There's quite a few implicit assumptions in that.

In my case, I am double-taxed (both Japan and US side) on capital gains. Tax treaties reduce, but not eliminate, the extent of double-taxation.

Many US-based brokers do not allow Americans abroad to purchase mutual funds, so VFIAX is not a choice for me.

Maybe I can go with eMAXIS Slim All Country... Oh, but that is a PFIC under IRS rules and I'd be taxed on unrealized capital gains. So I guess no Japan-equivalents of VT for me. That's fine, I guess I'll just buy VT in my US-based brokerage account; but now I'm in a suboptimal spot with respect to monthly contributions, calculating JPY-denominated income tax on dividends, etc.

I even made an implicit assumption when I said "calculating JPY-denominated income tax on dividends". That assumes your tax status is permanent resident. If your tax status is non-permanent resident, then a decent financial advisor will recognize that only the extent of income remitted to Japan gets taxed, so VT distributing at all isn't an issue (until 5 years later). What should you do before the 5 year threshold is hit? etc. etc.

But yes, if you're born in America and plan to stay within the same state for the rest of your life, then a 100% automated setup that simply deposits $1,000/mo into VFIAX is probably fine. (But keep in mind, to most non-Americans, VOO is not really diversified compared to funds like VT).

Otherwise, there is value in consulting someone (or something) that regularly handles taxes and financial planning.