I'm not a fan of Fortnite but its development is far more smoother and impressive than compared to PUBG. It being free, ideal for a variety of people and not too serious, makes it very popular and welcoming game. PUBG on the other hand has had its day - it'll still be successful going forward and be popular, but greed, lousy development and numerous issues really sour the overall experience.
Both games have been incredibly successful but on the face of it they are nothing special - just a popular mode (that's been around for 5+ years) that is becoming more saturated by the minute. Neither game has much scope, variety or depth and if it wasn't for the virtual marketplace on PUBG or the Fortnite cosmetics; not many people would care.
PUBG was brilliant in the first year but ironically when they 'finished' it; that's when it started going downhill whereas Fortnite goes from strength to strength as the more YouTuber's ram it down people's throats; the more it will continue to thrive until the limited Battle Royale concept fizzles out and people move on to the next big craze whatever that is. I don't think BR will be as prominent as survival was but it's certainly had a bigger and faster impact on the industry.
Not sure if either game is deserving of its overall success but they have both proven to be the right games at the right time even if they aren't anything all that defining.