I don't know if the rumors are true, but some people have claimed the peek insulator on the Troll RTA melted while dry burning coils. I've never had a problem with Wotofo products though, so I'm guessing they were glowing their coils insanely hot for way too long. Wotofo also claims to buy their peek insulators from Germany.
The Reload RTA is supposed to be am incredible tank. I've never gotten my hands on an original since they were like $80 but there are some decent clones out there (if that's you kinda thing). The Manta RTA looks like a great alternative to it though, I think I might be picking one up.
The Merlin Mini RTA you mentioned is great in single coil mode. I've used a buddies on a few occasions and really enjoyed it, but I can't speak on the duel coils.
The OBS Nano Engine is also great, esp if you want a single coil. I didn't like the other Engine versions as much though.
The Serpent SMM is an amazing single coil RTA if you like a slightly restricted drag.
A few more I've heard are good (but don't have first hand experience with) are the Voluna (duel), Centaur (single), and Boreas V2 (thinking about picking this one up as well.