It's a bit of both, isn't it? Many people have indicated that the ALC series has helped improve their game and enabled them to move up several levels of difficulty. Credit for which I insist on sharing with everyone who contributes to the threads.
Right now, as the Mehmed game seems to indicate, the biggest challenge I could face on Monarch level would be another isolated start. I'm not opposed to trying one out again, but it has to be with the right leader. Leaders who lack an early UU and who are not naturally warmongers are suitable for that type of start. Of those coming up soon, I'd say Peter of Russia and Roosevelt of America would be suitable to an isolated start if the RNG gods deem it appropriate.
Isabella seems to fall into that category, but in her case, founding an early religion and then spreading it seems like the best course of action, again making an isolated start a bit of a waste for her. And Shaka should definitely start with some victims nearby to get the most out of those Impis.
Ragnar, on the other hand, could definitely start isolated. Once his UBs are in place, Viking galleys with Navigation 1 could potentially hop to other land masses while his base remains out of reach until Astronomy, and his UU could attack directly from aboard ship provided the target is relatively weak. Fun! Then again, Viking galleys still can't enter ocean. Hmmm...
In fact, given Ragnar's many strengths (chief among them: he's Financial), I was thinking of moving that game up to Emperor. Either that or playing it against the BTS expansion pack's improved AI.