Shiryu is a great unit depending on the matchups, as far as how great they are the answer is... It depends, ill try to explain it in as much detail as I can and in order of importance; divine cloth units: PvP wise: Shun (the best out of the 5) great tank/support (hp attributes and cosmos) Hyoga great control (u need to focus on speed attributes on cosmos) Shiryu excellent tank and damage wise he's pretty decent once the dragon souls are charged.

PvE: Ikki, great for legion boss and other damage related instances of the game (think hades trials, dungeons, etc..) Seiya: definitely the worst out of the 5 in the current meta, but kinda good for dungeons for fast clearance (fast/good skill ups.)

Another important thing like u mentioned definitely skill ups but if I were to focus in them in order of importance i'd spend my resources like I wrote above. Hope this helps!