got through to onsite at RenTech last fall for a quant researcher role. not going to pretend i have the full picture because the process is deliberately opaque, but here's what i can say.
phone screen was with someone technical, not a recruiter. opened with a probability problem that took me about 20 minutes to work through. i don't think they care about speed as much as they care about whether you can narrate your reasoning and catch yourself when you go down the wrong path. i caught myself twice and corrected. they seemed to like that more than people who barrel to an answer.
the onsite was a full day. mix of math/stats problems, a coding component (not leetcode style, more like: given this data structure, how would you design an algorithm that does X), and then what i'd describe as an extended conversation with two researchers who were clearly probing for intellectual curiosity more than domain knowledge. they kept asking "and then what?" after each answer.
no behavioral panel in the traditional sense. they did ask one question about how i'd handle a situation where my model was performing well but no one trusted it. i think that's about as close as they get.
i didn't get an offer. the feedback was extremely vague, which i expected. but honestly the process felt more honest about what they were testing than most places i've interviewed.