ETF vs Mutual Fund for Americans (related to IBSJ new NISA offering)

Hi all. I searched previous posts and the wiki but was unable to find any information, so sorry if this is obvious – but why are mutual funds considered worse for Americans than ETFs?

I personally hold only ETFs (VT) in my Vanguard account, but I'm currently in the process of opening an IBSJ account to then open a new NISA and noticed the Tsumitate portion only allows mutual funds. Is holding mutual funds in this portion going to be a problem for US citizens?

Thank you for any info regarding why mutual funds may or may not be a problem for US citizens!

by redfinadvice