I'm not sure if the're OP because the're expensive as hell, but the're very annoying to deal with.
Screening tends to be a fool's errand because you need to screen everywhere but they only need a small opening to sneak in, and the constant battling will constantly disrupt your screen. Fleas might work though, since the're not only very cheap but also invisible when stationary, meaning the scythes run right into them and get detected.
My way of dealing with them is to build faradays around the most important stuff in my base. You do need multiple faradays do cover multiple angles, so it might pay to start building your base with faraday positioning in mind. Putting them atop little spires can increase their ability to fire over your stuff. You will also want to leave a few riots hanging around base to deal with them.
Then it comes to protecting valuable but squishy units such as a tremor, make a circle of riots/raiders around them so that faradays can't sneak around your line and backstab.
One strategy I haven't tried much, but is worth a shot, is cloaking your own things, including your base. Scythes rely heavily on their ability to see your stuff without being seen to plan their attacks. If you take that away, they might have no choice but to make blind attacks which greatly increases their chances of screwing up.
Even if you take all reasonable precautions, you're still gonna lose some things to an insistent scythe player, but you can win the attrition war in the long run.