I think Lava 2 rewards are still ok but not Lava 1... Totally sucks...
Guys, lava 2 rewards are 300% more than Mesa 4. But yes, lava 1 rewards suck. Skip lava 1 and go right to lava 2. I am the leader of a high ranked guild and we organize lava 2 and 3 through chat rooms we have created in "line chat" which is a free application you can download from the Google play store. If you would like to be invited to this chat, just message me on line chat. My screen name is : ajsbest
I will invite you to the lava 2 and 3 chat room where you can coordinate with other to run it. If you don't have ghoulem, cupid, pd, dd, vlad, and trenataur, and over half of them evolved, then you are probably not read for lava 2. If you do have heroes like this, then feel free to message me and I will invite you to the lava 2 and 3 chat. We have hundreds of members in there.
Lava Isle 1 is waste of time, don't bother, stick to mesa 4
Lava Isle 2 is harder to figure out than 3, I don't know any teams that farm 2
Lava Isle 3 is tricky to figure out, you need to have teams setup using line chat etc and are able to tweak things and go through a fair few fails to get it right, but once you do and can farm Lava 3, the rewards are amazing.
Yes the merits per chest are 50 less than mesa 4, but I screenshotted my rewards from the 2 purple chests I got and they were:
1400 shards, 1 talent refresh card, 1 monster pass and 5 blessed tome 2's (purplebooks).
Even the green chests give decent rewards, it's definitely worth your time trying to farm lava 3.
As for joining a random game, it wont happen and even if you do it will be a fail. Lava needs to be with set teams, set heroes and set crests and even then you may get a fail now and again as some of enemies have blade shell talents