I have two problems with Google. First, it misses a lot of legal routes while featuring routes closed long ago--many, many such routes. I have a Magellan 5 which is great for showing what's out there and it is great for downloading GPX tracks, too. There is no voice on trail, but I do get turn-by-turn visual which is great for trails with difficult route finding.
Second, I prefer to know about the trails beforehand, including scenery, length, surface, and, given the extensive custom Sonoran pinstripping I got in Week 3 of ownership, I started paying more attention to scratchy spots warnings than I used to!
I also use MVUMs but we found none available in Montana which was a bummer. There, I had to get out of two trails as they got too narrow and scratchy. On one of them, I barely turned back after a full mile and had an almost nightmare passing experience with a UTV and two ATVs on the way back. I am glad I got out without body panel damage. I actually got that trail from a local 4wd guide but it was written for dedicated trail rigs so did not square well with having a vehicle with a shiny paint.
Another thing I want to know about is traffic. Trails like the Alpine Loop, Backaway to Crown King, or some dirt roads can feel like Manhattan at times.
Last but not least, in forested parts of the west, Google offers a ton of in-the-forest dirt road drives, which are very pleasant but not something I would plan a day around.
This even happens in Colorado. Say, Piedra road north of Pagosa Springs, driven to the very end, is a very pleasant, longish drive. But it is inside the forest except for one section towards the end, which is scenic. I am very happy I drove it once (while family was still sleeping). Would I drive it again? I don't think so. Would I plan a day around driving it? Absolutely not. It is nothing like the Silverton trails and it ends abruptly at a barrier in the middle of a meadow, so there is no particular destination either.
For all these reasons--scenery, time, difficulty, traffic--I prefer to work from a guide. But I do understand that others may prefer to feel like exploring and figuring things out for themselves. Both ways are fun, but I prefer the former (for now).