I haven't seen him once. vs I have never seen him.

Some examples from the web:

• You haven't seen him once since you've been out there.
• Since that day three years ago, I haven't seen Sawatari once.
• I haven't seen you once.
• We've been together 48 hours, I haven't seen you call her once.
• You've worked here almost a year and I've never seen him once.
• This mill may be miserable, but since we got here, we haven't seen Count Olaf once.
• A land that everybody should have seen once.
• I've already seen him once today.
• I've only seen him once before.
• You must have seen him once before then.
• I seen him once or twice.
• We've seen him only once.
• I haven't seen him since we switched.
• I haven't seen him in four days.
• Also thriving and lush pastures promise breathtaking natural spectacles that can be seen once.
• I might have seen him in here once.
• I eat here every day. I have never seen you once.
• Yes, I have seen an angel once.
• I have seen a panda once.
• I have seen these eyes once.

Some examples from the web:

• To my knowledge, I have never seen him.
• I have never seen him before.
• I work in one of his companies, but I've have never seen him.
• I have never seen him more angry.
• On the day he should be happiest, I have never seen him so destroyed.
• The Pharaoh has mercenaries, but I have never seen one like him.
• No. I have never seen him be violent before.
• I have never seen him for five years.
• I might never have seen him again.
• i mean, he was just miserable with lana, and now I honestly have never seen him more happy.
• But I have never seen it.
• Your lungs got a Chicago Fire escort like I have never seen.
• People are suffering like I have never seen.
• No. This I have never seen.
• We could say as much today of the many believers throughout the world who love the Pope even though they have never seen him.
• There has been a destructive force unleashed on this town such as I have never seen.
• Something has stimulated Jake's brain functioning in a way that I have never seen.
• And I can personally attest to a quickening of emotional material since this last summer Solstice-the likes of which I have never seen.
• I would also like to take this opportunity to thank all those friends whom I have never seen.
• But I've never seen him.