There can still be legal challenges, but overturning any decision after the safe harbor date puts that state's electoral votes in jeopardy of being thrown out by Congress.
If, for example, after the safe harbor date there became available rock solid proof that the election results were wrong, I expect a state court would overturn the decision. Or, more likely, state officials would change the decision. Validate votes that may be a risk are better than invalidate votes that are safe. The courts have to weigh those options.
States courts are going to be very reluctant to put their state's votes in jeopardy. Federal courts are going to be very reluctant to put a state's votes in jeopardy against their will. The remedy could likely be more harmful than any harm caused.
It doesn't end anything, but it puts a big stone on the scale to not overturn any decision that has already been made.