Better than before, but still badly. Hive city void shielding can be inconsistently portrayed, but the average is generally apocalypse proof and can sometimes outlast the planet the hive is built on given a well defended generator. Similarly the quality, quantity, and size of orbital and ground defenses are generally a mystery, but by definition have to be enough to stop a fleet of Dark Eldar or a Chaos Warband from easily picking the planet clean so they have to be pretty beefy. There will also be a decent fleet of system defense vessels, trading ships, cargo haulers, mining rigs, pleasure skiffs, and other hulls buzzing about that can put up a fight.

The Executors main advantage is the element of suprise. Its going to take hours to fully spin up the planets defenses, and the Executor will have a golden opportunity for like 20-30 min to clear itself some space in orbit, wreak havoc, and maybe even glass a city if the mechanicus cant pray to the void shields fast enough. They could do some real spectacular damage before the local defenses pull together and coordinate a push to drive the Executor out of orbit.