The big drawback to pre-planned routes are just that, they are preplanned. You lose a ton of "adventure of what's around the bend". For me, that is big. If you need to go off-route from a organized route, you may not have done the research to know to avoid Highway X in favor of Highway Y. You may not realize the largest ball of twine is only 10 miles off-route and, assuming you are a twine ball enthusiast, that could be the miss of a lifetime. I am sick in that I actually enjoy creating routes and riding them some day. I have routed over 500 individual routes (though have only ridden about 40%) around the country so obviously I do not fear going off route . Heck, pay me 2 cents a mile & I'll do a route for you . Most of the routes are short, i.e. 50 miles or less, and connect to other routes, either mine or some organized route or road. Think of a spider web, not massive trails though some do connect lots of short routes into 1,000+ mile routes, i.e. Alexander, KS (TransAm) to De Funiak Springs, FL (Southern Tier) or Brownsville, TX to Winnipeg.
A major drawback to creating routes is that I tend to remove a lot of the surprises during the research. I already know what is ahead and like I indicated above, I enjoy wondering what's ahead.
Personally, if a map exists I will use it but will just as easily create my own segments if a map doesn't exist where I want to go. For instance, I have a created a few routes to supplement organized routes/roads for a "fall colors" ride I hope to do in a couple of years going from Montreal, QB to Burlington (mine) to Albany, NY (ACA/mine) to Erie, PA (Erie Canal, mine, & ACA) to Pittsburg (ACA) to Sheperdstown, WY (GAP) to Front Royal, VA (mine) to past Asheville (BRP) to Statesboro (mine) to Jacksonville (ACA). Even though the above routes lean more toward organized routes, I typically do about 75% custom. I have been touring for over 35 years so I have done most of the organized routes so either have to repeat or do my own now.
I rarely use a topo map as you can learn to "read" the topography of a map if you have some basic knowledge of the area. For instance, a squiggly road that follows a river is typically a gentle climb (or as gentle as the river falls). However, a squiggly line not following a river has a good chance that it is going up and over a ridge or pass. Occasionally, areas like the Ozarks throw you off as they just pave a straight road up and down cliffs it seems. You can also use Google satellite and streetview to get an idea if you really need to. However, coming from the west coast, anything thrown at you by GA will be doable. Also, do not be afraid to ride some gravel if needed. It isn't bad at all.
Again, have a great ride!