Respond from the Chinese players.

Units probably have more defense when they are on cities. Siege master takes away that defense bonus.

I am neutral this time because I am a Chinese and I also serve here. I will just simply post what they say. I think it will be a better idea that we based on their comments and something else we know about this game, and I will send it there. Is it fine?

I think it's great that we have contact with the other Easytech fan communities as this can help bring more game knowledge and maybe even start new friendships.
Having said that even within our own community there is often disagreement about what is best,and I am sure it's the same everywhere else and that's to be expected.

All game ideas and knowledge is fine,but the most convincing and the one that has the biggest impact though is the one backed numbers,facts,data analysis,game file hexes,and logic.Meaning in general it's much better to show how we arrive to a conclusion if we want to convince others of our arguments.In other words I will not simply state X,Y,or Z is the best,I will instead show you how I got to that conclusion.Not only will that convince you (if you agreed with me) but now you can make the same statement knowing you yourself can back it up with fairly solid evidence.

In other words,the "why" or "how" we got to think the way we do is often just as important as the "what" we think.

Is there any statistics from the chinese comrades regarding the real impact of that skill? Maybe I couldn't see solid damage difference... but I think it is not that great. Anyway even in case it is perceptible there is no logic in the opinion that Siege Master is useful while Infantry tactics is useless. Siege Master is situational skill anyway while those called useless are very important passive skills that work all the situations. 

Here're my opinions

Berthier vs Golytisin
Costs to upgrade Berthier is, yes, insane. In normal circumstance, I would not spend that for a geography skill. But if you play this game a lot, you will reach a point where you have all the items you need and other generals are fully upgraded so you will have no use for medals (thanks for the grinding method). At this point, I do not mind upgrading Berthier because he has better potential than Golytsin (warhorse grants him geography but also takes up one item slot) and I have spared medals. In fact, I am now searching for Agha to train him.
So, who is better? It's up to player's preference and constraint. If you are progressing the campaigns, you probably have limited medals so Golytsin can be a better choice. But if you want a good end game cavalry general (excludes emblem generals and princess), go for Berthier.

Surprise vs Mobility (and other varieties)
Surprise increases max dmg and Mobility adds min dmg. Statistically, they give the same expected (average) output so I do not think the claim that "Mobility is useless" is a valid fact, it is an opinion. The difference is the standard deviation (range) of damage; Surprise -> wider (high SD) and mobility -> narrower (low SD). As for my preference, I like a narrower range (low SD) of damage because I can predict my output more accurately and know whether I can make a kill. This is crucial for a cavalry general since you probably do not want a bad luck that a low damage would come out and make you miss the kill. A max rolled damage can be a waste due to an overkill but improved min damage is always useful. This is also a reason why I prefer Suvorov (with combat skills) to Victoria (with assault art). Anyway, a wider damage range means more uncertainty and it's uncertainty that makes a weaker general defeat a stronger one. So, in my opinion, Mobility skill (and their variations) is far from useless. It ultimately depends on player's preference.

Savary vs Ney vs Sulkowski
"Savary is better than Ney if they are not attacking someone in a city" is probably based on an assumption that Mobility is useless which, as I previously argue, is not necessary true. Fireproof is not very useful for cavalry since you can always move away and the fire damage is laughable (I normally consider it as a special effect to make the battle look...cool). Siege Master is situational but, as a cavalry general, it is quite common that you attack a unit stationing in town then capture the town in the same turn so this "situaltional" is more probable for cavalry than others. Sulkowski is pretty decent. Too bad, I previously did not notice him.

Accurate, Bugle, Strike
Accurate, Bugle, and Strike are great skills as I statistically proved that in other thread. I normally consider them the most important skills for each unit type.

Leadership Skill
Leadership skill is very good for a cavalry general who needs to get into the middle of enemy troops and makes multiple kills. This is why I think Lan is the best cavalry general. Though the skill has the same effect on every unit type, I do not see why you would bring an infantry/artillery general in the middle of enemies. Most of the time, AI is not smart enough to flank you unless you place yourself to be flanked. So in my opinion, it is a great skill for cavalry but a so-so one for other type of units. I so not count the fact that it allows the general to move when you run out of food as a significant benefit of this skill since it is way too situational and when that happens, you should rather revise your resource management than rely on one general with leadership skill.