How's the dimensions?. Does it flush on the side, top and bottom?.
I guess it depends on some factors. I have a Spigen thin fit so it doesnt have the biggest edges. But I applied the screen protector without the line up guide included and it came out good enough just using my case as a guide. There is about 1 mm on the side and bottom for me because i didnt use the included guide. But i think most screen protectors will have some space, and also i dont think the s21+ is completely flat.
If you use a bigger case the screen might fill in better.
Update: after using it yesterday it worked great after the lotion trick. The only thing that made it barely work was after i took a shower. Then it worked maybe 1/30 times. About 1 hour after the shower it still wouldnt work great about 1/10. But when i woke up this morning it was working 1/1 even using a light tap it worked.
Ill update again in a few days to see if it changes.
For reference My fingers arent overaly dry, I used lotion once then registered 3 different fingers only once. Each finger was registered in a circular motion. The lotion was only used to register the fingerprints.These are my results. Results may vary depending on your setup.