Here are some of the common complaints about Jill Stein:

As the Green Party candidate she will pull in more third party votes than most (if not all) of the other parties. While those views are usually trivial in number in the grand scheme of things, they can have a big impact in let seeing states.

She has not gone out of her way to criticize Trump, and has instead treated both Trump and Harris as being equally bad, even though Trump is arguably much more anti-environmental than Harris.

This isn’t her first time coming in as what’s seen as a Democrat vote-spoiler candidate.

From the perspective of some, she pops up every four years to try to take votes off the Democrat candidate and then seemingly disappears for another four years.

She has a bit of a cult of personality thing going on - arguably moreso than either Harris or Biden.

She’s in her mid-70s, and so no spring chicken. This has become more of a concern this cycle because a) she’s for years older than she was in 2020 (obviously), b) age has become more of a point of concern given how both Biden and Trump have apparently seen marked cognitive decline at less than 10 years her seniors.

There’s some evidence to suggest she is cozy with Putin, who is clearly does not have America’s best interests in mind. For example, she was photographed at a fancy dinner sitting at the same table as him and several of his ‘crew’ (among others). The common excuse for this is “she didn’t choose her seating!” But equally there’s no reason she couldn’t have got up and moved or left the event.

Photo of the dinner in question, with labels:

I’m sure there are other points I’m missing. But those are the ones that spring to mind.