What is the biggest problem
…is this a serious question? You’re asking about a 21 year old non-classic vehicle…It’s impossible to determine this over the internet.
The biggest problem are the rear control arm bushings. They tend to need to be replaced too soon.
But my 98 was extremely reliable. We finally got rid of it at close to 500k miles. But you’re asking about a 21yo vehicle with an unknown history. As long as it was maintained well and no accidents - it’s an excellent vehicle. Engine is considered one of the most durable V6 engines ever made. It does have a timing-belt and it is an interference engine. However the timing belt is pretty simple to change. A decent back-yard mechanic can do it in a couple hours. The hardest single part is removing the radiator.
I don’t think the 97 has rear control arms, it was made on the pickup chassis. IIRC, that was the last year they did that.
The 96 thru 2004 Pathfinders were unibody. They were NOT based on the pickup chassis.