@Jomini - for human-player any province is easy. Vienna and Praha is a bit more difficult... because HRE. Only this. Ottomans-human can quickly get the army 100+k. Kaizer + Austria/Bohemia AI cannot have (parallel) great army like a Stambul.
For AI ottoman or russian conquest of Oxford is impossible in early game.
In MP game I've never seen anyone conquered isles province owned by Player-England in early game.
The difference is that holding Oxford requires you to merely match the stacks of England, a state which is going to be distracted in France and have a hostile Scotland trapped on the same island. Allies, like Portugal, Austria, or Denmark are terrible at landing forces and you can off through them back into the sea pretty easily. Regardless of everything else, I have 15 years to consolidate Oxford, build out a navy, etc.
The mainland provinces, on the other hand, mean that powerful states and their allies will march large armies into your territory. It is virtually impossible to isolate the fight to just one decent sized state. Say I do punch out Vienna. At some point Austria will be restocked on armies and have allies (say something like Milan, Aragon, and Saxony); keeping Vienna until I had admin 10 requires that I find a way to deal with the allies as well. if I am particularly weak, anyone nearby (Poland, Bavaria, Hungary) may well go to war just to take over the place.
In multiplayer, I doubt this matters too much at all. If the OE or Poland don't get free westernization, they will just be dropped a bit in threat estimation and everyone else will adjust their expectations a bit (e.g. strong, Westernized OE = Spanish/Austrian/French accord to cheaply carve up Italy into respected spheres of Influence with strong pressure from Spain/Russia/Austria to take back territory from the OE).
For SP, the only AIs likely to Westernize are Poland and maybe the odd Hungary or OE. This is quite reasonable. What other AIs need somewhere else to Westernize?