Few things are, Sundowner's shield requires to be attacked with precision, aiming at the gaps in the shield and the arms that hold it up, not the shield itself. And Sundowner's shield can explode an infinite amount of times upon contact in a short amount of time, as opposed to the limited 3 the solar shield has. Other than that discrepancy, yeh can see the similarities.
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Animus Viral
Golden Heretic Snakeboat
Another way of looking at it is that sundowners shield can take minimum three precision hits (knocking 2 off with each blade mode strike), and while solar shield gets 3 hits max, it also regenerates, meaning that technically, it can still take infinite hits, although not constantly like sundowners can