Never make the mistake of assuming you’ve seen it all. There’s someone out there doing something ridiculous to make money. Or worse: providing a rationale for the ridiculousness.
Pop-ups, by definition, ruin the user experience. That’s it. If a pop-up gets opt-ins for an inconsequential newsletter that Gmail filters out anyway, fine. But don’t claim pop-ups are good UX. Let’s examine the arguments I’ve run into for pop-ups-as-good UX.

1. “If you design it well, users won’t mind.”

Umm… yes they will. But there’s a bigger problem: you’re now confusing UX design – a narrower topic – with user experience. From a strict UX design perspective, I’ll grant that you could design an aesthetically acceptable pop-up. On usertesting.com, the example of a pop-up on booking.com is used. I think it looks crummy, you can judge for yourself, but from a broader user experience standpoint, we’re at an impasse. “A good example from Booking.com of how to retain abandoning visitors with a popup without being too intrusive or distracting your visitors too much.”

“Too much” is a subjective judgement call you’re making on behalf of your users because you want the conversions. That’s not acceptable UX – unless your users have specifically told you during UX user research they enjoy pop-ups. Good luck with that.

This is the booking.com example of what the author calls a “tastefully designed” pop-up:

2. “As long as you don’t show them a pop-up upon entry, it’s ok.”

If I’m walking in a park, I’d rather step on dog poop at the end of my walk than the beginning. But all things being equal, I’d rather not step in poop at all.

3. “If the pop-up shows relevant content, it’s not annoying.”

All pop-ups are, by their definition, annoying. If your content is so amazing, so deeply relevant, I will eventually find it and I will probably do business with you. Maybe not today, but tomorrow or the next. If you have something truly great or indispensable on your site, all my colleagues will be sharing it, and I’ll be back.
Here’s the “relevant content” rationale: “If the content helps the visitor solve their problem, find the right solution, or get answers to their questions, the popup won’t be perceived as annoying.”
Perhaps. But I’ve never had a pop-up answer a specific question I had, so I can’t say. I’ve had plenty of web sites answer my questions, and if they do it exceptionally or consistently, I subscribe to them. What about this is broken that requires a pop-up guessing at my intent?
“Instead, it will be seen as helpful, or at least as intending to help, not disturb.” Not sure what UX planet you are living on, but helpful is not about being interrupted. It’s about being there when I need you. It’s about letting me easily escalate the relationship. Pop-ups are like copping a feel. They are completely out of context of the content date we are having. It will be seen for what it is: an aggressive attempt to secure my opt-in. If it works, then it works. I’ve bought products from aggressive salespeople before. But leave “good UX” out of it.

4. Personalize your pop-ups by visitor groups.

Two problems: personalized pop-ups are still pop-ups. I don’t care if you left the poop on my sidewalk specifically for me to step in. It’s still poop. Second: classifying visitors into interest groups is a difficult task until itself. My interests tend to be individual, so if you send me a group message, I know you’re guessing. Lumping me into an “interest group” isn’t so personal.
This supposedly good example ad of “personalized pop-up” also includes an insult:

If you’re going to insult me when I opt-out, at least give me a “Yo mama!”. That would at least crack a smile.

5. Users don’t care because they don’t complain.

Consider this a formal complaint. Your pop-ups suck.
More rationalizations: “Visitors don’t seem to really care about it at all. “We had absolutely zero user complaints” said the guys from WPBeginner.”
Well, first off, it takes some effort to complain properly in this world. An annoying pop-up is pretty far down on the complaints list, below an Internet outage, a cancelled plane flight, or a blue screen of death. Doesn’t mean it’s not irritating. Also, I have no business relationship with you. If you had a box on your site saying “do you hate our pop-ups,” I would have clicked “yes.”

6. We only show the same reader a pop-up once a month, so that makes it good UX.

I don’t want to see the pop-up once a year. And: you’re assuming I’m using the same browser with the same cookies on the same device. Nah, I’m gonna see that pop-up more than once a month.

7. Bounce rate didn’t go up, so we’re all set.

A bounce rate is a small piece of the puzzle. It doesn’t account for brand perception. I can tell you this: I am less likely to socially share and refer pages and web sites that have pop-ups. Where’s the data on that behavior?
More justification for user interruption: “Plus if I’m really smart I can actually use popups to enhance UX like Vero does. When you stay for longer than 30 seconds on their landing page and don’t take action a popup in the right corner of your browser window will appear asking “What is the main thing preventing you from signing up to Vero at this point?””
Thanks for letting me know, I’ll avoid Vero like the plague. That may be a useful data gathering and conversion tool. But that’s not a better user experience.
There is ONE thing I can think of that might improve my UX via pop-up. If the founder of your site – not a low-level service rep 5,000 miles away, but your founder – sent me a pop-up instant message asking me, “What do you think of our site? Did you find what you were looking for?” And then interacted with me thoughtfully. That might work. But that’s about one-to-one relationships – and the great user experience that stems from making such a connection.

The wrap – pop-ups may convert, but they don’t UX

Pop-ups may be effective from a very narrow view. If they weren’t, we wouldn’t see so many of them. But let’s view them for what they are. Dressing them up as “good UX” is a futile endeavor that obscures an honest assessment.