We give them all mountain, but there is a MASSIVE range for the servants in 7-A. Again, up scaling to back scaling and vice versa. Hercules can shit on a bunch of other mountain levelers, and Asterios is superior to him but Hector can pierce him with his NP. Romulus is supposed to be far superior to most of the other Roman servants including Caesar (it is implied him losing to Nero was because he was holding back, which makes sense), and defeated Lancer Artoria with moderate difficulty, who ******* one shot Mordred casually and ripped through her armor like paper. Ruler Servants are supposed to be capable of taking on most any other servant, but very in power between themselves. Spartacus can one shot most other combat applicable servants even without Crying warmonger, even those on the level of Cu and Archer EMIYA. Jeanne can also block blows from Spartacus, and is comparable to her Alter form, who Archer of Shinjuku back scales from, who can engage in combat with Saber Alter who is physically superior or equal to Hercules.
Do you see the issue? The ranges between these people by metrics of one shots, back scaling, catagorical extremes, and various combinations of these things. The reason for this is the skill of Fate servants is world class. There are mountain level servants who can take out Country level beings with skill and tactics alone. The range of them is massive, but can keep up with eachother by almost raw skill alone. I made a small scaling chain above just to illustrate my point, and as you can see it loops back in on itself. Most of what I mentioned above was all within the mountain level range as well. Do you honestly think we could actually quantify where all these people stand without accounting for skill. Servant stats at least allow us to know that some of these people are a good bit lower than others.
As for your Shiki point: Potentially, but we never saw the battle. Knowing how Shiki fights in the show, it was probably mostly skill, but had the stats to at least block attacks.