This is well known: https://github.com/zcash/zcash/issues/1705

The ticket says "In some cases, some transactions with joinsplits do not get mined." So this does not prove droffels point. He said all tx with only z-addresses could not be mined.

Any other source for this claim?

He's 100% correct, as is the GitHub issue. Transactions that were z-addr -> t-addr, or vice versa, were being mined, but private transactions (my z-addr pays your z-addr) were not.

Per the ZCash website (https://z.cash/blog/state-of-the-network-2016-10-31.html) -

"There is one known bug, which causes private transactions (those in which all of the inputs and outputs are shielded addresses) to not get mined."