Have not used OnX. Here in Maine, we still largely have a tradition of access to private land and sorting out the ownership is not critical, at least in the unorganized townships and more rural organized towns. And our state law is that access to unposted land, even for hunting, is allowed unless notified by the owner that it's not. That changes as areas get more developed, but I live 5 miles from the state capital and except for "safety zone" signs around houses, there is no posted land on my road.
I will make a plug for another GIS app, GAIA GPS. For an annual fee, it gives me USGS topo; NOAA Charts; state-owned lands; a public lands layer; a private lands layer that identifies the owner (though not everywhere in Maine), and satellite at several different resolutions. I think it's $40 a year.
The public lands layer was really useful when I was fishing in NW Wyoming in areas with a mix of BLM, Forest Service, State and private land. I ran into a local game warden and asked him if I could safely use it to find the public parcels with river access. He'd never seen the app before, but when I showed it to him, he said it was spot on in the areas he knew well.
As a GPS, the app is as good as any I've used. If you download the USGS and NOAA and other base maps, it works with great accuracy even with no cell service. I only wish battery life on my phone was better.
What other mapping apps are folks using?