A sickle bar mower will do a good job in nice standing have,if;
the sickle sections and guards are sharp and properly adjusted (tightly) so as to shear the stalks,
back when I mowed with a sickle bar I had two sharpened bars hung on the tractor to be swapped out in the hay field when they started having problems.
Then when you finished mowing for the day they all needed to be sharpened to be ready to go the next day.
If the hay was lodged or down they would not pick it up and would frequently plug and require backing up while lifting the sickle to clear it,
which only worked part of the time. So then it was shut down the tractor get off clear the sickle spread out the hay plug so it could dry as well as the rest of the field,
get the tractor started again and proceed till the next issue came up.
Hay mowed with a sickle bar mower also needed a conditioner to be pulled thru to crush the stalks so it would dry faster,
otherwise it would be on the ground for several days before it could be baled. Some areas with low humidity did not have that problem.
Also a sickle bar mower is slower then a haybine with a reel and sickle setup and much slower then a discbine.