Chen is better then most give him credit for, definitely still situational though. His strengths are his very low cooldown infinite use abilities and his ability to chase. He is extremely good in the early game and vs backliners who like to kite (li Ming, Valla, medic). His dive kick is 5sec CD (which is very low for a mobility skill), and at 4 and 7 he gets a better burning rage (which is typically a lvl 16 talent) and then a move speed bonus. So he just dives people and shits on them as they can't get away. He's not good in the current meta because the current meta does not rely much on those mobile backline heroes and he's weak to current popular front liners like anubarak. Chen is probably really good vs genji though.
Sonya does not have 8k health, most heroes don't reach nearly that, that's like chogal numbers. But it doesn't really matter if she has comparable health, she's not a tank because her kit doesn't really fit. She's all about going in hard to abuse her short CD damage abilities on squishes. She doesnt have good mitigation until 16 or 20 and those are on 60 second cd. She is beefy and is probably the type of hero your looking for, but don't try to use her in a tank role or you'll be setting yourself up for failure.
Sonya and Chen are actually very similar in role and power level. Although I'd say Chen is stronger early game and Sonya is stronger late game, and being stronger in the late game is better. Also Sonya is really strong vs popular front line meta picks such as anub, arthas, and Leo. ... well maybe not anub so much.
