An interesting article... pointing out some fringe cases in which selecting an appropriate index could make a difference, specially if you're looking at emerging markets.

Costs and weights...

The situation is reversed among All-World products that combine developed and emerging markets in a single index. The FTSE All-World ETF costs 0.25% versus 0.4% for the cheapest MSCI All-Country World ETF.

However, try to avoid mixing index providers in your portfolio at the broad market level. For example, if you combine an MSCI World ETF with a FTSE Emerging Markets ETF, then you won’t have any exposure to South Korea or Poland. Meanwhile, you’ll hold both those markets twice if you take the FTSE Developed World ETF along with an MSCI Emerging Markets ETF.

https://www.justetf.com/uk/news/etf/...-provider.html