The thing about any electric GPS device is always going to be battery life. I have fallen foul of this once in the past on an "easy" walk. No matter what devices I carry, I now also carry enough battery backup for double the amount of time I think is required. I also use a solar charger on multi day hikes.
I am definitely a belt, brace, piece of string and some gaffa tape type of person. Phone with VR, simless phone with VR, ancient garmin gps device, compass and relevant maps. Spare batteries for garmin, big Anker battery pack for phones and solar charger (multi day only).
My phone in normal mode will use well less than half its charge a day running VR in track mode. Don't forget, if you have "Location" selected on in your phone settings, the gps receiver is running anyway. My gps uses 2 AAs and they last ages. My normal night time routine includes putting my mobile on to charge so I never leave home with less than 100% charge. I check the charge on my gps at the same time as I outload a route to it. When it is down to 2 out of 4 bars, I swap the batteries. I have a small ziploc bag of spare AA and AAAs for my head torch and radio in my pack all the time. One lesson learned is to check them from time to time. I swap batteries in the field so rarely they have been known to go off in the bag.
My gripe with the OS app is that you cannot record a track while following a route.
General gripes with all phone based nav apps:
If you have any standard phone security, for example mine has fingerprint or PIN, it is a PITA to have to activate the phone each time to use the app.
Phone screens can be hard to read in bright light
The screen switches off after a short period of inactivity to conserve the battery and you have to reactivate it.
(I know I can change this in the settings but I prefer it that way for normal use to conserve the battery and protect my security. I do not want to keep changing my settings every time I take it for a walk which is why I prefer the gps - long battery life and screen always on).