Based on what I've read, they're not that effective. ATACMS have been used throughout the SMO in Ukraine, with hundreds of launches against Russian targets, and Russian Air Defense was capable of handling about 90% of them in 2024. The ATACMS missile is no Iskander.
Even the announcement is basically a NATO declaration of war on Russia, tho, based on what Putin said previously. And even one ATACMS getting through and causing damage in Moscow for instance would be a significant provocation, and would likely lead the Russians to target NATOs Intelligence/Surveillance/Reconnaissance infrastructure, or even targeting sites in Europe. I don't know if they'd go so far as to target sites in the US.
It's all pretty crazy! The Russians have been very careful to slowly grind away the NATO forces in Ukraine so far. That may change with this ATACMS announcement, based on Putin's warnings about responding to long-range missiles - he has also said that when a fight is clearly going to happen, it's best to punch first. So who knows at this point.