It's only a bigger boost if you have more than 24 units. I never do multiplayer (is it still awfully buggy?) and you only need 20 units to dominate the AI...as long as 9 of them are artillery.
You are playing the AI. The AI is not, I repeat, NOT representative of a real opponent. Which, obviously, is by far Civ 5's greatest shortcoming since 99,9% of games are played SP.
An Autocrat without 50 - possibly many more - units will never happen.
So you're saying that autocracy and order are much better in MP? I only have SP experience.
That's your problem right there. Without MP experience (or, experience against a strong AI, which we unfortunately don't have) you get a distorted picture of the amount of units needed to conquer anything. Against proper opponents - multiplayer - you need a substantially larger and/or higher tech army to even stand a chance of taking a city. Conquest is
hard against a defender who knows what he's doing. Just look at how easy it is for you to defend against the AI despite it outnumbering you.
You really should try to play just a single game of MP - if for nothing else then to get a more realistic idea of the game balance.
Not only have I done this, but I found that freedom is a great way to have a huge science, gold and hammer output from a core empire. I dealt with the happiness problems by razing non-essential cities.
You know, I could beat the game without using a single policy. That doesn't mean Freedom is useful for conquest - just that the game you're playing is too easy (see above).
However...freedom is the only late game policy that you will likely finish. Order and autocracy are geared towards military and large empires. Late game, you need a strong cultural output with fewer cities in order to get later policies. Neither order nor autocracy support smaller empires, and unless I am mistaken, you would be very lucky to get even 3 policies deep in either of the last two trees. In comparison, freedom allows you to have incredibly productive cities that can be used for anything.
Freedom allows you to have a smaller number of highly developed cities. However, Freedom empires cannot match the sheer military might and production capacity of Order and [succesful] Autocracy empires. The real benefit of Freedom empires is they allow you to have superior culture, or rather, a larger number of social policies. It is also very easy to hold you ground in a Freedom empire due to increased city strength. Basically, Freedom allows you to be a small empire that the others will want to leave alone because trying to attack you is too much trouble, allowing you to pursue your cultural victory.
It is also incorrect that you are unlikely to finish Order or Autocracy trees - it just won't happen very quickly. Expect finishing either tree somewhere in the 20th century which - tadaaa! - is when you are also supposed to be when you complete them according to history. Both trees unlock only in Industrial Era for a reason. Once again, I think you have a distorted idea of how easy it is to win a game due to how bad the AI is. You are not supposed to be able to win ie. a Domination victory prior to at least the Industrial Era - more likely you'd require nukes or Death Robots for that. Essentially, real games against real opponents are going to require much more effort - and time - to win.
The difference between Freedom and Order + Autocracy empires is how they gather culture. Freedom has a few cities filled with nice wonders + Piety and Freedom boosts. Order and Autocracy empires are rather more based on the average level of your basic culture building (monument - Temple - Opera - Museum - Tower) since your culture per turn approaches that of your current basic culture building level per turn for very large numbers of cities. They will get their policies as well, but it will (and should!) take much longer.