Even if the color production wasn’t an issue, it still makes no sense as a 5th mox. Combo decks need to be as lean as possible to avoid fizzling and if you’re running a card that doesn’t actively contribute to the combo you’re wasting deck space. Cheerios needs Opal because it’s a RW deck that needs red mana to combo and in that deck it has no drawback. I given the options for moxen in Modern I can’t think of another true combo deck that’s true for, though I’m open to being wrong.

Ah, doy, you cannot tap for blue with Sram as your only legendary.

This is an interesting idea. Does the card slot into Cheerios as an additional Mox Opal copy that only works when you draw Sram rather than Puresteel? You need the Opal to cast multiple Retracts.

Similarly, is this card good enough with Baral in storm? It’s likely too inconsistent there, especially considering the creature is already a must kill target.

A very interesting idea. Ruinous Blast is a very interesting card and will likely see some kind of play. As for the rest of the deck, my question is whether the payoff is really worth the price. You’ve identified a number of problems with your current build, but my problem is card quality. A lot of the legends you’re running are really weak for Modern and if you’re engine isn’t running, can the deck stand on its own? Now, Reki is potentially powerful alongside Mox Amber, and that suggests that there is power to be found. Rather than going the value route that your post and the list suggest, Reki may be better utilized as a combo piece though what the win condition would be is beyond me. Drawing through your deck like Glimpse of Nature is very good, so if you want to go the legend tribal route, maybe look into some king of combo kill.

For the Whir or Tezzeret decks, you’re already running Opals, do you really need another mox, especially one that’s harder to turn on? The overall problem is that the cheap legends that aren’t Thalia are pretty weak, and after turn two any color that want’s acceleration has access to Talismans or Mind Stone. Yes, they cost mana but also don’t require the deckbuilding constraints needed to make the mox work. The question you have to ask is whether the need for additional mana really justifies a spell slot in your deck since Amber Mox does not replace lands. At the moment the cost/benefit ratio is just not in Amber’s favor, but you’re right, maybe something will come along that makes it all worthwhile.

Or more generally planeswalker decks. Again I doubt its good enough but things like gideon of the trials will turn it on – tribal gideons with kytheons?

I do think you understate the value of a turn two accelerant that isnt a creature. First, only green has birds, second birds gets bolted a lot. You can sandbag the opal and drop it turn two for a burst of mana.

All in all its probably lacking a real modern shell at the moment, but red and white are in the artifact synergy business and the cheap legendary business so it may just be a matter of time before we get the toolcraft exemplar dream deck.

Since the spoilers of Dominaria, I made some work building a modern deck around Mox Amber.
You ‘re right about Mox Amber being just a mana acceletator, with no very good potential in a legend aggro decks. But what if we make Mox Amber and all of our Legends draw us card with Reki, The History of Kamikawa. I’m working on a deck that relies on draw for fast board domination, including the new : Kamahl’s Druidic Vow XGG Legendary Sorcery and Urza’s Ruinous Blast , that in my opinion I think they are very good in the right build. I made some tests with these sorceries and combined with Jace, Vryn’s Prodigy these spells look sweet with no actually negative. I had some insane draws (but also often dead) with Reki in play, and a lot of lands with Kamahl’s Druidic Vow that helped me play an Urza’s Ruinous Blast from the graveyard with Jace. However the whole deck looks very vulnerable cause the lack of hatebears , protection, and the lack of a good win con (my currently is Ajani-Gideon).
I hope that Dominaria will give us some good legendary Hatebears.
So, take a look if you want in the deck and tell me your opinion.
https://tappedout.net/mtg-decks/amber-blue-vow-ajani-gideon/

Thank you for your time.
Κeep up the good work.