Stryd can work great during ultra events that take many hours. The battery of Stryd can last up to 20 hours, and will continue to pass data to a watch or phone during that time. Since Stryd itself can only record up to 9 hours worth of running data, it cannot store any run over that time (but your watch likely has that capability).
PowerCenter can accept data up to 8 hours (either directly from Stryd, or from another platform like Garmin Connect or Movescount). If your data is longer than 8 hours, you will not be able to import it fully into PowerCenter.
The development team is aware of this limitation, and is considering to address this is a future release. The workaround is to analyze your data in Garmin Connect, TrainingPeaks or other platform.
If you have additional questions, contact firstname.lastname@example.org.