GPS is just a device that pulls in the signals the satellites send out and figures out your coordinates.
The maps are what could be an issue since that's pulled from the internet. But the various map programs let you download maps ahead of time.
The difference is the app you can pick the steps individually, specifically picking an arrival time without filling out the rest. The website you have to do everything else first before it lets you pick a time.
Reviews are inherently skewed negative. When I read Reviews for any tourism related service, it's usually pretty easy to filter out the unreliable reviews