I think cellphone as a plot device can be strong, and yes it probably doesn't make much sense in a fantasy world of sword and magic.
HOWEVER, there needs to be more to that. And all the authors have to do is put themselves in the shoes of the person being blackmailed.
For example, imagine a beautiful woman (think Emma Watson) blackmails you with racy pictures of you buying a whore and threatens to show it to your girlfriend unless you fuck her silly. What do you do?
You say HELL YEAH let's fuck. But you don't go "omg Emma's tight pussy is soooo much better than my girlfriend". Hurrr can't live without it. You don't go "heroin addict" mode over it. Sure when you're fucking your now "average" looking girlfriend, you think about Emma's hot face and tight cunt. And how she can throat you dick better than your girlfriend. But you're not going to go "I cant cum anymore from my girls' pussy".
You'll feel guilty leaving your plain Jane childhood girlfriend, but you get super mega hot movie star. Now you might say "Wow that's great, but where's the NTR feeling?"
Exactly...This is why most NTR villains are ugly and fat, unless they put YOU in the shoes of being the plain jane girl.
There must be repulsion, so not Emma Watson blackmailing you, but think Melissa McCarthy. But then it's different view right? You're not going to continually seek out Melissa McCarthy's pussy even if it's better. In fact, if she keeps blackmailing you, you need to try to get that phone. You need to FIGHT because you don't WANT to fucking her even if she IS better than sex. Even if she does give you the sloppiest blowjob in the world where your girlfriend doesn't even lick the head. 99% of NTR mangas / games miss this FIGHT. They instantly go into "Dayum she's Emma Watson, so hot, so good, fuck my current girlfriend" mode.
That's no way to write NTR, why? Because the protag has fallen so fast and she/he is no longer beautiful. Would you still love your wife if she spread her legs after getting dicked by Henry Cavil and leaves you? No. She's just a cheating slut. There's nothing to corrupt. She's black and broken.
A realistic NTR means she has to constantly keep reminding himself/ herself of all the beautiful memories you had together, vs Henry Cavil or Emma Watson's hot body.
A good NTR should also have the villain be morally corrupt (cheats on the victim afterwards, uses her / him like a sex toy, etc), otherwise you'll see little wrong with it unless it's POV is the person that loses wife or husband.
Instead of these complex, realistic life choices and thought patterns, we get dumb shit that reads more like regular cheating. The girl that gets blackmailed doesn't fight back, she's mindless, brainless, and useless. She's not seeking to get out of her predicament, even if she's suppose to be smart and beautiful. She just accepts being used like a rag. Nor does she think "Would Henry Cavil be a better partner than my current bf, or is she just using my body?" Nope. Nothing beyond the superficial sex, which is unrealistic, boring, and quite frankly elementary.
Like hey, this could be be fun.
