My advice would be to weigh up how many people seem to be negative. If it's only one or two, it may be just them......if it's more than that, it's a red flag to me.

During interviews, Managers/Directors/HR etc are likely to build up a picture of the company that makes it attractive to you. They want the best of the bunch of interviews they carry out, to join them, as such, they might say stuff like "you'll start on this salary, but it'll quickly increase", or, "There's lots of room for progression", or "We'll be looking to train you up in this area, or this area".......all of that could easily be the truth, but when it comes to actually working there, you could find that there are a million reasons why that isn't happening for people. They've not signed any guarantees of what's been promised, or how well they'll look after you. "Your salary will go up quickly" can easily become "We just want to stabilise our growth a bit", or, "We don't think you're quite there yet", or similar.

Unfortunately, people will tend to be negative if they aren't being looked after, and if they aren't the only one complaining, there's a good chance it's not their fault.

If I turn up somewhere and there's a lot of negativity in multiple parts of the business, you can bet that I'll have my CV knocking around with various agencies, just to make sure I've got the seeds sown for a way out should I need it.

TL:DR: When hiring, companies will be very positive to make the position seem attractive. They won't tell you the really bad stuff. If you arrive and there's significant negativity, it should be a red flag.