occsionally, Yi Sun Sin. some allies are extremely useful, while others are just...Sardinia 1798:/
Edit: I've been playing around with countries in 1798 (almost beat them all. just have Ottomans, Sardinia, and Spain) and am quite familiar with which ones are good and which ones are bad for at least this era (If you're on their side, obviously)
Sardinia: Always dies, unless your it. Cannon Fodder
Portugal: A coin flip, really. They can make amazing progress, especially if helped. Alone, however, they're mostly just more cannon fodder/stallers.
Spain: Useless. Portugal will do well for themselves and usually take Madrid by about...turn 30-40? I think. Need to be put down early-mid game.
France: Meh. They can do well, but also tend to have a spitting contest with England. Just hope they harass the enemy enough for your generals to plow through what's left.
Nederlanden: Meh. They need help at the beginning, and do nothing afterwards. They rarely help me. You can easily steal their land, though:P
England: Great. They always make progress in France/hold a moderate defense against them. They also have people all over the map, so they tend to help in most areas.
Italy: Decent enough. France/Ottomans tend to help them slightly, and Napoleon often kills Naples. They'll definitely be fodder, of course, but good fodder!
Naples: Eh. They sometimes kill Napoleon which means they have a fighting chance. That's a coin toss, though. Keep an eye on them and help if possible, but don't count on Naples.
HRE: Good. They always hold their ground long enough for you to help, and often make several pushes all over the place.
Prussians: Not bad, not good. They usually hold their own in the same manner as the HRE, but the Prussians rarely ever make much progress after a few turns.
Denmark: Coin flip. They kill Dobeln, great! They don't...Well, Dobeln is Dobeln. You can ignore them for a good long while, though.
Sweden: Same as Denmark. Dobeln makes or breaks them. #DoblenTheImmortal
Russia: Great. They will harass the Ottomans while dealing with the Polish. They sometimes march down to France and all that, as well.
Ottomans: Not bad. They can cause a decent bit of damage and stall a number of good generals. Posh, or whatever his name is, is surprisingly resilient, too.
Poland: Meh. If you're near them, then yes. Cannon fodder/territory you can steal when recapturing. If you're not near them, then they die. Always. Can't be saved. Poland, who?