They're very solid, but they're just unpopular. In my case I don't use them much because they're fairly weak in halfs. Of course, then the downside of mono is that you will have a bonus that does nothing the majority of the time. They're situational, and on strong SoA weeks like this(Rolph unbanned) you'll see them at higher elo.
A few reason:
1. They aren't overly great in a half deck (splash is good though) meaning they're more effective in mono
2. They lack a powerful nuke that most other clans have, even their 5* cards that are actually useable lack damage
3. The 5* that are actually good either have weird abilities for their star count (Such as defeat) or simply lack damage (Minerva)
4. Their 4* are lacklustre with only Dr Falkenstein being a 'great' card.
ie: Upper half of star count is very weak/ doesn't support the clans needs