Old question but I find it increasingly relevant as the trend keeps growing. I can hardly visit a site without having modals interfering with my navigation.
The unsolicited and premature sign up/survey modal is now similar to the cookie consent message. It's so systematic that a lot of people don't even read it and they look directly for the close button. The difference is that it is even worse with newsletter subscription because those modals often intentionally block you from reading anything on the page.
One particular nasty practice is to make the cancel/close button as hard to find as possible by using faint colors, small icons or deliberately unexpected positioning. I usually leave those site immediately because it tells me the owners don't respect my freedom. It's like of a shop owner opening its till and locking the door behind you as soon as you enter. It's creepy and overly mercantile.
This trend is motivated by researches showing higher conversion rate with modals. But how many people are actually put off by it and bounce? How many want to unsubscribe after the first newsletter?
The technique reminds me of the pop-up ads in the late 90s. Website started overusing it and it became so annoying that users started to install plug-ins to block them then browsers integrated the functionality as an option and eventually blocked them by default. Now we see ad blockers and other types of extensions detecting subscribe overlays and I wish it could soon become a standard option in browsers to opt-out or at least standardise the notifications.
This is yet another sad example of a web design trend driven by marketing short-sightedness rather than usability. It is cyclical: it will disappear and re-emerge using new technologies. Whether it's bad usability or not doesn't matter, you'll have to design it anyway, but if you do making it less intrusive, less forceful, easier to close (e.g. scrolling threshold, escape key, big close button) and letting the user taste your content before first would help a lot.
Another thing is to avoid nagging the user, remember their answer and allow a long period before asking again. I have seen sites where the same modal appears on every article or each visit.