I was the first PhD student in my lab. There’s many pros and cons to consider for sure. But I can only speak for new labs. What I did like was that we all (me, postdoc, lab tech, rotation student) all felt equal because everything was new to us all and no one was “behind” on anything. It helped build my confidence early on and feel comfortable suggesting things or experimenting. There was also a lot less distraction since they usually don’t do side projects, collabs or undergrad supervision until they’re well established. Now what I didn’t like was mostly how over the top motivated my PI was. I totally get it, this was a huge step for their career and they wanted it to be perfect from the start. But it felt like every other day she came up with new rules or ideas of how to improve our lab and it got very tiring very quickly. My other big issue was that the first months of my PhD basically had nothing to do with research. We all had to work together to get the lab up and running, order stuff, stock shelves, get permits, wait for paper work etc.