I reckon with a diversified stock-heavy portfolio in iDeCo something like 5%+ a year is probably possible. That might be conservative given historical performance. You also have direct control of your money, and it belongs to you.
Nenkin kikin, on the other hand, is out of your hands. It is horrendously complicated (check this page out and see if you can understand it without getting a headache: https://www.npfa.or.jp/system/type_benefit.html). I haven't done the sums, but paying 7,000 a month for 40 years in order to get 20,000 yen a month in retirement might not be the best, particularly if it's not index linked: https://www.npfa.or.jp/check/table.html
I think the risk of the government changing the nenkin kikin rules or formula is higher than them deciding to confiscate or tax iDeCo.
I will get round to writing a new blog post about this at some point though, and will actually do some calculations for that