http://riecoin.org/: ..
In Primecoin there is no practical way of estimating the time before finding a block, moreover difficulty 10.1 is easier than 9.9 making
it impossible to estimate how secure the network is.
This is not correct. At diff=9.9, all 10-chains found are counted as blocks. So a 10.x is always more difficult than a 9.x diff.
By the way diff a.x is more difficult than a.y f x>y, for any a as expected.