There are two sides to it all really. Rockstar aren't guilt-free of this as they carefully marketed the game. Even silence is a tactic that marketers use and Rockstar played it very well with V. That said, there are a lot of people who definitely did overhype it. A statement in 2011 that the game is their most ambitious yet doesn't hold a candle two years later when they've had time to cut features and realize their grand ambition wouldn't work. I could start a book today and say "it's my most ambitious novel to date" and end up publishing a short story (not that I could publish a story but you get the idea). It's quite sad that there isn't really a black and white to it. Rockstar did some things but fans also expected way too much in some cases. Generally though, I think the game is well liked enough. Most of us admit it has flaws but still enjoy it. There are a few loudmouths who absolutely hate it but that happens all the time.
Oh, those gta youtubers also pushed the hype. Confirming the tiniest things and then they give there opinion about it. Some guys went crazy in the comment section.
But R* promised the best parts of their best games, ecosystem for example and a "MASSIVE" map.
Tell me when did R* said that? Link, source, proof.
I believe Rockstar did say that the map was bigger than SA, IV, and RDR combined... which when scaled properly ended up being true.