All of this information I am about to provide is somewhat dated and may not apply to the current meta, but I played a lot of Uba Stax back in the day. Here are some comments:
The real strength of Uba Stax is not Welder. It is Workshop. There are two kinds of hands that are keepable. There are Workshop hands and there are Welder + Bazaar hands. Everything else is pretty bad. The problem with Welder + Bazaar hands was already addressed by Soly. You are leaning hard on a 1cc 1 toughness creature that doesn't do anything the turn it comes into play. In many metas this is a huge no-no.
Uba Stax should not be thought of as being built around Welder. Welder is a nice like to have card. It is something that costs you 1 mana and your opponent must answer it. It is not a card to build your whole strategy around.
The best card that your list is missing IMHO is Serum Powder.
Serum Powder allows you to aggressively mulligan to a hand with Mishra's Workshop or Sol Ring. You do not do this quite as aggressively as Dredge mulligans for Bazaar, but you should mulligan a lot.
General advantages of Serum Powder in Uba Stax are:
- It is a free one sided draw7 that can be played on turn 0. (AKA it is just a good card in general)
- Tapping for 1 mana is actually really good. Opening up with turn 1 Mox, Shop, Powder, Resistor is usually a strong play as you can play pretty much any card in your deck through Resistor the following turn.
- The 3cc mana cost is totally doable.
- The fact that it is a near dead draw in the mid/late game is heavily mitigated by Bazaar. Just pitch it to Bazaar.
- It Welds out for better artifacts.
- It Sacks to Smokestack.
- It taps to Tangle Wire (if you play it)
- Karn turns it into a 3/3 beatstick.
A big disadvantage is that it is shut off by Null Rod.
Null Rod is a really good card. We had a long debate once about the pros/cons of running Null Rod. I remember I was initially against it, but warmed to it as the debate went on. Now that Chalice of the Void is restricted that makes Null Rod better in a few ways. Chalice@0 already shut off Moxen, so Rod was weaker. Chalice@2 shut off Rod.
What I did not like about Rod was that it would shut off these artifacts that my opponent would either be sacking to smokestack or tapping to Tangle Wire anyway. Also Null Rod shut off a bunch of my own artifacts (including additional Null Rods) so I thought it was hurting me nearly as much as it hurt my opponent.
As I mentioned earlier the key to Uba Stax is the Shop and not Welder. For that reason I am against Sundering Titan. At 8cc it is significantly harder to cast than other options. I used Karn, Silver Golem back in the day. I did not use Null Rod at the time, but Karn was a fast clock when I needed it, ate Moxen and was much easier to hardcast at 5cc.
I remember concluding that Mana Vault was not that great and that was without Null Rod in the deck.
Consider cutting Null Rod. This might be a TERRIBLE idea now that you can only run 1 Chalice of the Void, but at least explore it. I want to be clear that I am not suggesting you actually do it. Only consider it.
Also consider adding Serum Powder. This only works if you mulligan aggressively to Workshop.
I would recommend dropping Sundering Titan.
Other cards to consider:
Expedition Map: Finds Bazaar, Workshop, Strip Mine, Academy or Barbarian Ring. Note that if you run Serum Powders the 2cc activate cost becomes much easier to cast.
Null Brooch: This combos well with Ensnaring Bridge, Uba Mask and Bazaar. Each tends to empty your hand anyway.
