Market rate is tricky. Especially for jobs that require less common skills. Some employers, small shops mostly, actively can't or don't bother to compete with the big tech shops on salary even though they recruit from the same pool of employees. They compete on life style, company culture, and other intangibles. If this is done openly and employees know what they are getting into it is a good situation as long as that open relationship is maintained. The employer and employee need to have a little trust and finances need to be open at least a little bit so the employees can see where the dollars are flowing.
It is a difficult position to be in, but I think the solution is relatively obvious. If you think you are being underpaid and you don't have an open line of communication to your employer about your compensation that is the crux of the problem. If you can't comfortably discuss compensation at any time with your manager in a no judgment zone where the employer immediately assumes you are leaving then you and the employer lose. At that point you may feel the only option is a competing offer and the employer may counter, but they will be counting the days until you are gone.
A middle service could help keep things fair, but any employer you would feel comfortable using something like a `my-market-rate.example.com` service report with you probably already can just have an open discussion about compensation without any concern. I could see it being useful in some places where the organization / manager really does just lack information about fair compensation but... that strikes me as pretty uncommon. Managers tend to know far better than employees their value in the wider market and to the organization specifically.
Ultimately, you just have to do your research and if you know you are underpaid you should be able to find employment somewhere else. If you fear a comp discussion would trigger your employer firing you, time to look for another job. If you think it won't change much, time to look for another job. If you think you are close and have a good relationship with your manager, just talk about it. In my experience employees, especially in high tech, are very worried about discussing compensation and how that might be perceived by their manager. In reality many folks that generally like their place of employment and end up where they come in with a counter offer could go back in time 6 months and have a direct conversation with their manager and get a raise (or not, signalling it might be time to move on). If it takes that competitive offer to budge your employer it means they know you are worth more but they weren't willing to give it to you fairly.
Anyhow, that is my brain dump on how these things play out.