It took us 3 months to get ECO Compiler to work with 20 ECOs. Most of the tricks and effective ways to use it aren't documented. It's a 20 Questions game to find this info. We regularly got fatal errors and it took a full day to isolate and package the error before a Synopsys engineer would work on it. Very time consuming.
Directives live forever.
We'd sequentially process ECOs and any old directive from the prior design would mess with the design currently being ECO-ed. One case generated bad logic with two gate outputs tied together. Synopsys has the test case. ECOs that involve hierarchy changes won't work. There's no feedback about alignment statements being accepted or doing anything.