I haven't got my hands on Psionics myself so far (the new resource management system takes some time to learn. It took some time until i realized that you have to collect the montlhly fundings yourself this time instant of getting them instantly added to your balance) but I kinda find the Sectoids too op.
They were kind of forgetable in EU but got a small comeback in EW when they introduced Mechtoids (Sectoids inside a Mech) who can get a powerful PSI Shield from their allied Sectoids.
In XCOM 2 however Sectoids got a big buff, maybe too big. They can panic soldiers, they can control them, they can irritate them, so they loose movement range and aiming power, they have (compared to other early enemies at this point) tons of HP and to make it worse, they can raise already dead people (enemies as well as your own soldiers) as PSI zombies (who act exactly liek the zombies from the same game except they won't burst into a scary alien spider thing).
In my opinion this is WAY too much for a single enemy. Especially since this is an early game enemy that appears in the first missions (the first was in my second mission I played).
Of course you can resist almost any of it as long as your soldiers have enough willpower (which they won't have so early in the game) or you enough mindshields (which you won't have because resources).
At least, when you killt he Sectoid, you will end all ill effects he caused and unlike the first game you can instantly use the mind washed soldier again in the turn you saved him. In the first game you had to wait another turn, leaving him in a particular unsafe spot.