Correct. Drifting snow will get in pretty much everywhere. A good 4-season tent will do a much better job of keeping that snow out. Rain is much less of a problem if the tent is well designed, i.e. an areodynamic shape where the wind won't just lift the fly up, leaving the inner tent exposed to the hammering rain. Or with vent openings that acts as funnels for rain and snow as soon as there is a little wind.

A very important consideration is also: How hard is it to pitch that tent in bad weather and high winds? The difference is HUGE among the tents at the markets, and this is something you really must pay attention to when reading those reviews.

For the reasons stated above, I am extremely sceptical for any construction where the load bearing construction is the inner tent, and where the fly is just something that is clipped onto the inner tent+poles. I assume there would be ways to design such constructions to be weather robust, and probably ways of pitching them in bad weather (without soaking the inner tent before the fly is in place). Untill I've actually tried it and being proved wrong my gut feeling is that this really isn't a good way to make weather robust tents. A robust load bearing "outer tent" where the inner tent is something that dangles inside it is what I prefer.

Another important consideration is ventilation. You will have condensation and rime frost in ANY tent, but much less of it if the ventilation is well designed. This requires a system of vents so you can have air flow through the tent. Even more so if you run a stove for cooking or heat. And you want a reliable and simple way of shutting those vents in bad weather. Preferably from the inside.

Do you plan for running a stove inside your tent? Then the height and volume of the vestibule is important. Many people think it is absolutely crazy to run a stove inside a tent, but the benefits - particular in bad weather - outweights the risks, IMO.

My winter camping experience - including some not really nice weather episodes - has mainly been in big tents with a wood stove. So, like you, I have very little hard earned experience on the subject. But (like you) I've assembled some knowledge through second hand experience of friends, reviews and forums.