There are four main factors that make an attack dangerous in diplomacy:
- The relative number of units attacking vs defending
- Adjacency of the territories you are defending
- How many territories there are surrounding your territories
- How many sea-land transitions the attacker has to make
In general, more units will always beat fewer units given enough time. A 2-on-1 in the early-game of diplomacy (when everyone is roughly equally sized) results in the death of the 1 unless the political situation shifts. A slower death is only less "dangerous" in that it gives more time for a political shift and thus increases your chances of gaining allies or of the alliance against you falling apart.
The defensibly of Warsaw + Ukraine + Sevastopol is as much about the adjacency of these territories as it is about the fact that Austria and Turkey only have 4 territories to attack from (Galicia, Rumania, Black Sea, and Syria). This limit in attack surface limits the number of units they can actively use to support moves, and thus means you can hold indefinitely with 4 units on the defense. It's actually even more defensible if you lose Sevastapol, as then they have only a 3-territory attack surface (Galicia, Rumania, and Sevastapol). Of course, if Germany allows them into Silesia, this situation changes.
To see why attack surface matters, try defending Galicia + Ukraine + Sevastapol from an attack by Austria and Turkey. This position is vastly harder to hold as it has a 6-territory attack surface even without Germany's territories (Bohemia, Vienna, Budapest, Rumania, Black Sea, and Syria). Galicia will fall almost immediately to a committed attack.
France, similarly, can defend against Italy and Germany more easily than Germany and England because Switzerland reduces the attack surface on France's Eastern side, where their Northern side has no such protection. While England does have to overcome a sea-land transitions to attack Paris (making the attack harder), they still threaten 3 of France's 5 supply centers (counting Spain and Portugal) by sea, which is sufficient to cripple France.
In conclusion, being attacked from countries on different sides is dangerous as a result of the increased attack surface as much as for the inability of your defensive fronts to support each other.