i use both... some of the issues people mention with flexis also apply to long lines btw, you can injure yourself and others with them, badly made or badly maintained ones snap... and the jolt at the end if you have a bolter is worse because they’re longer.
With a flexi, I use them for dogs who aren’t getting offlead (for various reasons) I find them easier to use like a really long lead and much easier to get a dog back to you on.
But you need to make sure you know how to use the lock, if you’re properly locking it rather than just holding it, always double check it’s on, but one that’s good quality (flexi branded rather than a cheap copy) you want one that’s designed for over the weight of your dog by lots and a tape not a cord one... and you want it to be clean and dry when you put it away.... and never touch the tape, only the handle when it’s unlocked.
Long lines I find are better for dogs that could be offlead if their recall was just a bit better - you can leave them trailing on the ground and just stand on it when you need to.
Oh and I use both on all sorts of sized dogs, largest would be about 45kg...
“ if you have to wait for the mechanism to reel back in and then lock it?”
Lock, quickly release the lock move forward then lock again, or, pull the dog towards you and unlock then lock again...it’s actually much quicker than trying to loop up a longline and even with a dog trying to get places it creates enough slack to reel in.
I wouldn’t use either near roads or other people or dogs and you always want to use them with a harness that had a loop on the dog’s back.