Both had, and Sakkari has, great careers, with glaring big issues. With Sakkari, whom I don't follow that closely, it's the finals. She obviously looks extremely fit, but you kinda have to wonder if the stamina is really there. Or maybe it's just the head, I don't know.
Elina had an inspiring career of overcoming her lack of weapons AND lack of clutchness. She was always glaringly nervous when things were close and rivals were raising their level. Only when the rivals were confused or tired she was ruthless. She played some great tennis in the tournaments she's won but in the end she also relied on opponents fatigue. The finals issue though never entered the picture for her, I wonder if it's cause she won her first WTA final when she was young and defended it year later, or if it's just because she truly had an extremely good stamina compared to the field. And a somewhat mind efficient game - she was a bit more freewheeling, non-robotic or technical ball striker, improvising often while not being in the initiative either. It was a really fascinating mix and different from other defensive players. But when ballbashers had their day it didn't work. Or creative players who were on game, she was played by them. I cannot forget that Krunić match in Bejing, the turnaround was insane.