I’ll comment on “stormworthyness” in general – so snow, wind and rain.
Wall angles are a big part of “stormworthyness” but there is a trade off here because shallow walls are good for shedding wind, while steeper angles are better for shedding snow. One important thing is for a tent to have consistent wall angles since a tent with one vertical panel and one horizontal panel would struggle in both wind and snow. A tents only as good as the weakest link. A shelter like the MLD Trailstar has consistent panel slopes but they are very low angle so they are great in extreme winds, but below average for snow loading. The X-Mid also has consistent panel slopes but at a medium to slightly steep slope.
Snow
The SS1 has some steep panels that would shed snow, but then two flatter panels on top that wouldn’t (depending on how sticky the snow is, but my experience with the SS2 is that it usually sticks). Similarly, traditional mids have lower angled walls going to the ends, so again snow sticks on these and collects at the head/foot of the tent. This shortens the effective length and strains the tent. Conversely, the X-Mid has consistent panel slopes with no low angled panels, so I think it would easily shed snow better than the SS1 and a traditional mid. Also, the X-Mid uses polyester which doesn’t sag, whereas nylon sags under wet snow loads because it absorbs the moisture and his exacerbates catching snow as the low angled panels of the SS1 and MLD Mids sag and become more scoop like. But all of these shelters could handle a couple inches of snow, so I view the differences as largely academic rather than practical in the context of lightweight backpacking.
Wind
The side panels on the SS1 are at a shallower slope to the X-Mid side panels, and these long sides of the tent are where the most wind catching area exists. So by default I’d give a small advantage to the SS1, although the X-Mid has two extra stake out points spread out along the base of this long side, so I expect you could easily reduce the force per stake to less than the SS1 if you used these. With both shelters, it would be ideal to point the narrow end into the wind and use the optional stakes if winds are high. It’s also worth mentioning that the X-Mid pitches easily in high winds because there are no vulnerable steps to the pitch (see 1:33 of my video for an explanation), whereas the TT recommended pitching method the SS is  hard to do in high winds – but there are other ways of pitching it that I work better (stake the full base first before adding poles). I would say these tents are similar overall.
With regards to the X-Mid vs a DuoMid in the wind, I think both shelters would be plenty stormworthy for any reasonably encountered conditions. And I am also not too sure of the answer since it is complex. The X-Mid has more side area since there are two peaks versus one, but it also spreads that force over two poles versus one. My hunch is that the X-Mid would do a bit better in high winds for the reason that the sides are shorter so less “wind scooping”. What I mean is that the distance from the bottom hem to the peak (on any side) is less in the X-Mid because there are two peaks and any given side is closer to one of these peaks that it is the single peak on a one pole mid. So there is less potential for wind to blow in the sides into a scoop and catch more wind. But then again, the X-Mid walls are a bit steeper so this may totally offset the former point. Perhaps tipping the balance back towards the X-Mid is that it has peak guylines which can be optionally deployed and these do a really good job of anchoring the peaks and can provide a huge assist to the steeper side walls in high winds, while traditional mids don’t have these.
Rain
It’s hard to imagine a better tent for rain than the X-Mid. A good rain tent is going to pitch fly first, have plenty of vestibule space for wet gear, have a door that protects the inner from falling rain, have a door that is big enough to enter/exit without getting wet, great venting and have a fly that doesn’t sag in the rain. All of these tents are good rain tents as they satisfy most of these criteria but I think the X-Mid would be the best because no sag poly is amazing and the vents are well above average. When you’re used to waking up after an all night rain and having the fly stuck to the mesh somewhere, switching to poly is amazing. The tent looks just as good in the morning as it did when you pitched it. The X-Mid also has great vents (far more functional than the StratoSpire vents).
Overall I would say the X-Mid would easily win in the snow because it doesn’t have any low angles panels like the SS1 (roof panels) and traditional mids (end panels), and would also be the best in the rain. Wind resistance is perhaps similar in all of these tents and some pretty advanced testing would be needed to identify the differences.
Of these, I think rain performance is the most important in the sense that it is something is often encountered and performance here can make a real difference in the quality of life on your trip. Whereas high winds and snow are less common, and all of these tents would fair well enough that differences here are perhaps more academic than practical.
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