There's nothing wrong with krill oil as part of a joint formula, but generally speaking you need 1100mg of an omega oil, not 300mg, so Flexuron gets off to a pretty poor start.
Then it commits the cardinal sin of joint health supplements, hyaluronic acid, which works great when injected into the site of the joint pain, but doesn't actually survive the stomach very well. Injected into the site 10mg can be fine, and a lot of oral supplements seem to get this 10-30mg number from these studies, however, orally this is not nearly enough HA.
There simply isn't enough space in the 30mg for it to be in it's effective dose, which needs to be closer to 200mg [1] with the lowest positive study being at 80mg, but either way it's not a great inclusion for Flexuron as it's still somewhat up in the air as to whether or not taking it orally works at all. This is where they get the claim that it's 3x better than glucosamine and chondroitin sulfate, but, glucosamine hcl (a purer version of standard glucosamine can make this claim too). And of course, the assumption would be reliant on one study that compared the two with a much larger amount of hyaluronic acid.
This means were left with only the astaxanthin which has been debunked by as many studies that have shown it has any effect on joint health at all. [2] That doesn't mean that it doesn't work at all, just that the jury is still out. It may well be the case that it works somewhat [3], but relatively inconsistently. There's a host of ingredients that we'd have liked to see in a joint support, things like gingerols, curcumin, glucosamine and chondroitin, reveratrol, vitamins, bromelain and more that are all far more well backed than astaxanthin.
And that's about all there is to Flexuron, there are much better joint health supplements on the market, and sure Flexuron isn't very expensive, and if you've got a mild omega fatty acid deficiency then you will likely get some benefit, but you'd be better off taking a better dose of omega 3.
References
1 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4729158/
2 - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3280376/
3 - https://pubmed.ncbi.nlm.nih.gov/32648603/