she is not a good actress...

I'm sorry, but she just isn't. She can never sell any emotion (or lack thereof) other than crying and looking sad. As Cameron in The Sarah Connor Chronicles, she never once managed to convince me that she was a cold, unfeeling machine. Whenever she spoke, it always felt like there was a hint of emotion she was actively trying to hide. She could never once sound neutral with her inflections or tone of voice.

When she was on Arrow, her acting was even more forced when she was brought on to play Deathstroke's daughter. Like with Cameron, she just couldn't convincingly play someone who is calculating and detached in her thinking. Emotions just seem to light up on her face every time she spoke. She honestly looks and sounds like a little girl doing a line reading.

Her acting range seems to be incredibly limited. She was okay as River Tam, but that's only because the part matched her real-life personality quite a bit. Summer seems every bit as dewy-eyed and childlike off-camera as she is when she's on. She's perfectly bangable, but she is just not a good actress, I'm sorry.