I didn't do 5:2, but I did the 8/16 for about a month plus and I'd say it's the least difficult and least torturous diet I've been on. I don't mean torturous in the sense that the hunger was unbearable, but it was a more comfortable diet because it didn't strictly restrict what you could eat, only when. For reference, one of the other diets I've done was the keto diet (the super low-carbs diet) and as someone who loves carbs and sugar, it was horrible - I ate a single grape after like 2 weeks of almost zero sugar and I literally burst into tears because it was so sweet and I felt so overwhelmed with happiness LMAO.
Anyway, back to IF, personally I saw very little weight loss but I saw definite fat loss based on the fit of my clothes, so for me IF was very much a fat-burning diet (which is pretty much the best kind). I feel like my skin cleared up a little as well? But idk if that was just imagination or a good month for me lol. I think the most important thing when it comes to IF is to avoid over-eating during the times you can eat bc you tend to be under the impression that you're eating 'normally'. The easiest way to do this (for me) was just to cut out snacking all together (I'm a serial snacker lol) - since I did the 8/16, during the 8 hours I could eat I strictly ate breakfast, lunch and dinner only so I knew I wasn't overcompensating for the 16 hours of 'fasting'.
But anyway, I didn't do it for all that long so all I can say is that in the short term it's a moderately effective diet with no negative side effects