About 10:1 noise:signal.
This comes from a variety of experiences: I used to manage a bug bounty for a mid-size company on Bugcrowd; in 2014 I surveyed people managing a bunch of programs across different sizes; I've participated in bug bounty programs for companies of different sizes.
The more you offer for rewards and the more recognizable your company name, the more you will be spammed by people submitting reports like (I kid you not): "You have the OPTIONS method allowed on your site this is really serious." The last time I looked at the numbers, Google had over 80,000 bug bounty reports per year, with about 10% of them being valid and maybe another order of magnitude being high severity (I'm fuzzy on the last bit). It's probably over 100,000 per year at this point. It's not uncommon for recognizable but smaller companies to receive one or more per day.
I'm aware of full-time security engineers at Facebook and Google who do almost nothing but respond to bug bounty reports. It's a lot like resumes - people who have essentially no qualifications, experience or (most importantly) a real vulnerability finding will nevertheless spam boilerplate bug reports to as many companies as they can. Take a look at the list of exclusions on a given program - you'll see that many of them explicitly call out common invalid findings that are so ridiculous it's kafkaesque.
HackerOne and Bugcrowd provide a lot of technical sophistication to prime companies for success, but there is an organizational component that is very difficult. If your program is very active, it requires dedication to tune it so you're not flushing engineer-hours away responding to nonsense. This is not to say they're bad - quite the opposite, I think they're fantastic. But I generally recommend smaller companies set up a vulnerability disclosure program through a solid third party, and do so without a monetary reward until they can commit to dealing with a reasonable deluge of reports.