Neither of the two Silvas I had lasted longer than 4 months and by the time the third one was due to come I told them to keep it and bought the Icom which it still going. The hassle of continualy having to strip down the console to change it and it failing in awkward places was too much. I actually ordered the navtex at the same time as the radio but 8 months later it hadn't turned up and it was cancelled as the same time as I returned the radios. The S12 that I bought as the backup radio failed the first time I switched it on and went back with the S15. I am not a big fan of Silva
I have the Garmin Blue as well and reckon it is probably the way you have mounted it that has saved problems. Mine works fine in very calm weather or going slow but doesn't work in any other conditions and when I mounted it in accordance with the instructions, hardly at all!
However I have talked to others with garmins and feel it is the transducer that is the problem, the higher powered ones like the Blue have a very large one and this seems to disturb the smooth water around the head and screw up the signal.
With yours inside the hull you don't have the turbulence that you get with transom mounts. I remounted mine far deeper into the water than Garmin recommend and this has made a fair difference for the return.
I probably would buy a Garmin sounder again but only if it didn't have such a huge transducer..................................