I think a lot going on here is that most people arenāt used to facing Yunan as much as Guin. Everyone raiding in diamond has an anti-Guin or anti-GM strategy that theyāve refined. On the other hand, you donāt see Yunan too often.
I know this is getting off topic, but I have just as good of results (if not better) with Aeron as tank as I did with Delilah or GM. I think people just arenāt used to attacking Aeronā¦
Uclapack and Zero are both known for their love for Yunan. Oh, Zero, my hero, why did u leave the game? 
I switched from Guin to Yunan and have the same bottom ceiling on trophies but my high ceiling is much higher with Yunan, today ended my raid run at 2730ish and then my defence took over. I peaked at 2860 purely on defence without any offensive raids and maintained that for about 2 hours before dropping to my standaed 2600ish range.
I will keep my Yunan defence and run Guin on wars.
Thing with Yunan tanks is that you need to have a very optimized setup for him to work.
Iām just going to drop this link right here:
I agree that green tanks seem the most uncommon color in Diamond. There are likely a number of reasons for this. Hereās my best guess on two of the bigger ones.
To run a Yunan tank its almost imperative to have atleast 1 Grave on your team.
I run Grave - Ariel - Yunan - Grave - Alaise and will be squeezing one more grave inside once I finish leveling him.
The trick to his defence is to stack DoT hard hitters like Grave with Yunan reducing healing and stacking his DoT on top of Graves.
At tournaments, when is rush attack, Yunan is a real nightmare.
Richard is able to hold more cups overnight than Elkanen. Probably because most of the diamond players could easily burn the forest with their fire Graveys and Marjanas.
agree. Elkanen is fast but in defense more a marathon runner than a sprinter due to his self healing effect he can last quite long.
My teammate who siting in low diamond
Sheās tough, but is her skill very useful when sheās very likely to go first?
Clearly itās working for your team mate, but itās surely not her best position?
Swap her with Isarnia and donāt sacrifice her on every battleā¦
Donāt have either, but even Boldtusk, Wilbur, Scarlett + Azlar as the only 5* already win at least 95 % of the fights against Elk tank teams for me. He is pretty sturdy, but doesnāt hit hard enough and also lacks unpleasant special effects. Therefore, attacks have much time to build up specials without being in danger.
As you said, Richard is way harder to fight without the right heroes. First, his strike really hurts and also has a very undesirable effect. Second, apart from Hansel, green 4* heroes arenāt great attackers. As for the TC20 5*, only one out of four is really powerful. If you donāt have Lianna or green 5* special heroes, Richard is a hard nut to crack.
As for green tanks in general: Apart from what has already been said, the fact that there are several awesome red 4* heroes might also play a role, especially in lower diamond regions whete people donāt have many 5* or super rare special heroesā¦
Tarlak isnāt a good tank. Nor is Hatter - he needs heroes charged to steal their specials, and tanks take the brunt of tiles.
Yunan is the only regular green tank in diamond, and heās reasonably rare given the difficulty in pulling him.
I use Liana in the right corner, and North in the left. Red tank. I have her offensively emblemed at +7. Sheās still valuable.
That sounds like sheās covered in nazi swastikas or lewd graffiti 
Sheās got a tribal above her bottom.
Tough to get heroes - Yunan / Albi / MN / Morgan
Accessible heroes - Elkanen
Groot is not so bad. Donāt just look at the tank itself. But rather look at who else can you pair them with.
Also, good tanks punish bad boards or stall time as if a time stop was activated.
Is that Yunan on tank position?
Yunan works great at tank and also right wing.