I have been using Pedometer++ since it’s debut on the iPhone 5S. I don’t think I’ve ever had it lose any significant amount of data and since every new iPhone was restored from a backup of an older model, it’s count should be pretty accurate. I continue to use it today, on both my iPhone 11 Pro and Apple Watch Series 4. The counter recently displayed 15 million steps.
It took me a little over 6 years and now, dear Underscore David Smith, I need a new goal.