There's also indication, a possibility, that for some reason koreader uses more battery when it works than nickel. Check this post:
https://github.com/koreader/koreader...ssue-175174643
So, it can be issue how power efficient koreader is over nickel when it works.
But, suspending koreader is (or was?) bigger problem. For me, using koreader has much more advantages over using nickel, so I don't even think about nickel, even if it is more power saving than koreader.