In Castle Age, Scorpions are a viable unit.
They are on par with Mangonels and can be helpful against Xbows and Cavalry Archers.
However, in Imperial Age, they are not as useful.
The upgrade costs more than 2,000 resources and the range stays at 7, which makes Onagers and Arbalests outrange them.
This makes Onagers a strong counter to Scorpions, as Arbalests can still do decent damage to them due to their slow speed.
While Siege Engineers can help, Onagers without Siege Engineers still hard counter them.
Arbalests also do extra damage to Scorpions in Imperial Age, making them a difficult unit to use.