That would be probably the relatively slow confirmation time, it's just too annoying when there are spam attacks going and the network and blocks getting full, it's just too frustrating to wait for the 1st confirmation that can take even hours.
Just pay higher fee. Problem solved.
it does not solve the problem every time as even with higher fee, I have seen many transactions taking lot of time to confirm.
blockchain is a good technology, but it sucks a lot these days, this ledger is an advantage on one side and on the other, a curse.