Hard to compare them since one is turn-based while the other is real-time. Also not sure how you would measure skill in either one, but I would say at least mechanics-wise this is a bit more complicated than your average TW game and therefore a bit harder to master in that regard.

I would say FoG is easier due to it being turn based, which means you have time to think. But the game has enough depth that multiplayer still rewards skilled players.

In singleplayer I think FoG is more challenging due to the complexities of total war make it difficult for the AI to pose a challenge.

For the record: I haven't played Total War games as much, as FoG 2 (I've played almost every TW game, but usually I got bored after a campaign or two, never played multiplayer).

What I like in FoG in comparison to TW games:

Terrain has much stronger effect, in my experience. In FoG 2 if you try to charge with heavy cavalry into the forest or a swamp, there is a good chance they will lose, even to peasants or archers. Even heavy infantry struggles in difficult terrain and can lose to weaker, but lighter units. In TW games, as far as I know, cavalry gets some penalties in the forest, but they aren't that big.

Attacking from the flanks or the rear is more impactful, at least than in newer TW games (older ones, like Medieval 2 were more similar to FoG, as far as I know). Again, in FoG 2 even peasants or massed archers can be devastating, if they charge enemy unit from the flank, especially if that unit is locked in combat.

In FoG 2 there are less rock-paper-scissor mechanics. Usually, when you get two units of different types, A and B, there are some scenarios, where A will defeat B and there are some, where B will defeat A, depending on terrain, positioning, etc.
For example: normally, a unit of heavy spearmen will easily defeat crossbowmen in open terrain in close combat. In rough ground, crossbowmen actually have the advantage and will probably defeat the spearmen.