Every one I have talked to seems perfectly happy. Some installed vessel view under the cowl. And had a few issues if they placed it in the back where the engine block and wiring interferes with connectivity. Other than that, I have heard no one complain at all. Only issue I can think of is that iPads are not waterproof so rain or sticking a wave would be an issue, and many smart phones are also not water tolerant.
As far as NMEA, in thinking back, I believe Don (Don Weed, BBC) had mentioned humminbird rather than Lowrance as having the problem, but that was a year ago. You might search the merc 3.0 forum on BBC, this comes up every now and then.