I used to use an ipad mini with Navionics on my rib. It was OK, and yes, cheaper than a built in unit, but the main problem was daylight readability. In the sun it was hard to see. Also you only have touchscreen controls, which are impossible to use at speed in a rib. Also you have to take it off when you leave the boat. I Eventually fitted a 9 inch Garmin, and it is really much better. As for a back up, I have Navionics on my iphone if the fixed unit ever goes down.
Just to add though that I also charter sailing boats in the Med, and I consistently find an ipad with Navionics is better than the chartplotters built in.
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