Yep, playing solo is fan, but also excruciating.
And i would say even with Germany it is quite a difference. Ottomans will fail miserably even on normal difficulty, and pretty much guaranteed to surrender against harder difficulty, and AH usually not really strong, sometimes struggle against even Serbia, and if they goes to war with Italy, you either help them out or they will surrender quickly.
But the French run was the worst, ok, veteran and triple alliance is a bit much, but Russia did not even tried against Germany, a few corps just stopped them, barely managed to take Galicia and then quickly pushed back. The UK retreated in every opportunity of counter-strikes, so the northern part just given up to the Germans, and even managed to lose its fleet to the German Navy.