Its two different things to arrive during the day and expect a shuttle and to arrive late evening and expect a shuttle. Travel at night in Central America, is much different than travel during the day when things are busy. Getting in the evening a taxi is usually better for sure, no matter where you are IMO.
But the poster asked about getting to Antigua and said nothing of a evening arrival. In that case, the shuttle works for most people, its simple, easy and efficient.
As for Guatemala City, like most large cities in Central America, for a newbie can be a tad intimidating, they tend to be hectic, gritty, dodgy and requires some street smarts, and knowledge of where to go, and where not to go, whether its here or San Jose Costa Rica. Nighttime, a taxi is a must in the city, and going out at night depends on the travelers, where they are going, and where they are staying.
The same can be said for Antigua, nighttime is the greatest risk for a foreigner/traveler, comes with the territory.
I don't know what "bad things" the poster heard, I have been on this forum since 2001 and can't recall a traveler having an issue in the last few years, so what is "bad" is all relative. My wife and I walked the main areas of the city the weekend before Xmas, around the stadium, the Xmas Market, the Pedestrian plaza, the main Plaza and many churches with no problems, in 2010/11.
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