For non-pretty things not made to do pretty things - Aimtech Warhammer Tactical Full Rail Shoutgun Mount w/Magpul M-LOK System

I got this for a couple reasons. One, it looks cool, I wanted rails to put iron sights on, and it also came with shell holders on the side. I opted for five.
The metal construction feels solid. It mounts to the barrel up front, and in the back a screw replaces the trigger pin. So it's on there. Really on there.
It's cut nicely to go just around the ejection port, so of course no issues there. And the rails are on-point, not too wide or too narrow.
The shell holders each have a good chunk of rubber in them making it very snug. I haven't had much experience with shell holders so take this with a grain of salt, but I do have to put some effort into getting them in or taking them out.
Now, I've kept my standard forend (so far) as I suspect aftermarket forends for these won't fit, at least not easily because of the MLOK slots up front that dip down. If I did, I'd look for one that flares outward, or something. Guys with bigger hands or longer fingers might have a harder time getting used to this as you can easily catch your fingers. Not quite a garand thumb but kinda similar. Maybe wear gloves.
One other gripe I have is, well the metal construction means it does metal things, like scraping and scratching softer materials like the finish of the shotgun. For cleaning, disassembly, or just for fun, I've taken this off multiple times. It can be a pain with the screws and needing tools, but as I said, once it's on there, it's on there. During these sessions, I've left quite a few marks. I don't really mind as it's a Mossberg and if anything adds to its character, but some people might be more particular about having a weapon with a clean finish.
Also a quick note: On a Mossberg 88, and presumably a 500, it does cover up the serial number. Just something to keep in mind if you're concerned about that sort of thing.
I think if they offered an option of it being a bit longer to cover more of the barrel and pushed the MLOK slots further up, it'd be a better product overall. Yes you'd be gaining a bit more weight, and you're messing with whatever harmonics you're trying to save (I mean who uses a shotgun for range and accuracy? It's already attached to the magazine tube anyway) but it's (likely) to open up options for forends and saving fingertips. I also wouldn't mind if the shell holders could come off individually. As it stands, my shell holders are one solid piece, and I presume the four-shell and six-shell variants are each one solid piece as well. I wouldn't mind if this could be customized further so you could vary how many and how far backward/forward on it you could mount your shell holders, like pushing it a little further forward so your folding stock could fold all the way.
So, yeah, I like it. It adds some versatility to a robust platform, while providing an option for shell holders and plenty of rail estate (hah, get it?). But as it's solid, it does add some weight and while your thumb and fingertips may suffer when cycling, I mean, it's going on a weapon that wasn't made to do gentle things. For something this robust, it also goes on very easily. It's not perfect but for a shotgun that's manages to be pretty cheap, I think it's a good buy.