Self contained GPS so ,any functions aren't dependent on your fish finder. As far as spotlock accuracy, there are many factors that will effect your results and it also depends on what you want to accomplish with it. You have to use the correct size TM for your boat. Too small or too large of thrust will effect your results. The size of your boat and how much it gets blown around will also have it's effects. Wind and wave conditions will also have an effect. For me, the stronger the wind/current the more accurate it is. If you want to sit on top of a brush pile and fish it vertically then you might not be happy. I bass fish and just need it to hold me in an area so that I can cast and I find it more than sufficient for that.
I fish a 15' boat which gets blown around. I couldn't hold it in a position with a manual TM better than the spot lock. I also don't have to worry about the boat control while doing other things. In deeper water, the only other option is an anchor but I'm not in one area long enough to justify dropping anchor and pulling a ton of weight up all day long. I have never regretted my Terrova and on any future boat that or a Ulterra will be my first upgrade.
If in doubt, take a ride with someone who has one and test it first hand before spending the bucks. You'll see them sold used by people who didn't and di not like them.