Lowrance LMS-520C how good are they for a backup unit?

[Sea Horse]

Just checking on your through of a Lowrance LMS-520C fish-finder sonar and mapping gps as a main unit or a back up. Want to do a little exploring down the Hudson River and down the east coast to Maryland. I do have a hand held that has never failed me, but would like to have a back up. I do chart my self and check with the hand held. Please give me your ideals.

Thank
Sea Horse