Sihil, really? "Destroy enemy artifact" tag costs around 1 supply. Lets see, Sihil costs 10 provisions, and starts at Ping for 1, in perfect conditions it can get to 6 power value in 3 turns. So yes, if it keeps killing cards from then on - it is too much value. But lets see, what it takes: A long round, enemy who cant be bothered to carry artifact destruction, continious supply of enemy units that can be actually killed with it and no active boosting counterplay from opponent.
And all of that assuming it doesnt just get blapped, leaving you with 3-5 power deficit and 3-5 provisions deficit.
It is a very high risk card that can indeed be good in a deck tailored to use it, with alot of supporting damage, nothing more.
Imlerith is a closer call. To be specific, imlerith + Eredin ability. Sitting at 9 power and immune immediatly after he enters the board, round is gone unless you can overpower 9, and by a margin, since hes gonna have support damage. Scorch is the only reliable countrerplay. Row-wide effect and random effects work, but they dont instantly shut it down, and monsters player not going to push him into Igni range. Still i wouldnt call it OP since you actually need to win 2 rounds, and Eredin ability has 1 charge for a game, and there are counterplay options. They just arent as cheap and simple as for Sihil.
If i had to rank by power alone, Its actually gonna be 3 cards, Witchers trio. They quite simply an autoinclude at their current cost. 12 power +2 deck efficiency (wich is rare nowadays) for 18 provisions? Yes, please, all day, any day. Thats disregarding that witchers have syregies now, and fairly good ones at that.
However if we go into "No counterplay, why is this even in tha game?" territory, it has to be either:
Usurper - dont build deck around your leader, or take an autoloss.
Viper Witchers and Kambi - thats a nice win condition you've built your deck around, would be a shame if something would happen to it, and you had no way to prevent it.
In conclusion - Witchers trio are probably the most OP thing in PTR. There are just some cards that arent Overpowered, they just shouldnt exist alltogether.