Asset allocation- Global portfolio

I am hard time understanding the implementation hurdles for the global portfolio- particularly the paragraph beneath Home country bias on page 35 vol 3.
I understand that dividing the real estate etc. into potions is difficult- but isn’t it the purpose of the ETF and mutual funds- right?
I do not get the point here. Can anybody clarify for me?

It’s all true about hurdles.
Look from this perspective. There are several global top-level asset classes - equity, bonds, real estate, commodity. For simplicity, we are not considering now crypto and other staff.
So, calculate total equity capitalization - not easy, but doable.
Bonds - pretty difficult due to giant universe. Some providers can offer a portion of this information, so with a precision we can accomplish it.
Commodity - holy ■■■■, how it’s hard to count. Gold, silver - ok, look into investable ETFs. What about timber and others?
Real Estate - oh, man. REITs are about 2 trillions of dollars only or a bit more. The vast majority of real estate - e.g. residencials - are non-marketable and uncountable. On the other side, many of us have their own property and don’t have to make additional investment into this asset class