I assume you're talking about the 135 hp V6 that Merc made in the mid to late 80s. I'm not aware of any problems with them specifically.
I've never known a Merc to eat a coil pack, although I've replaced a few on OMC engines. I've replaced a couple of switch boxes on Merc V6 engines with the trigger/stator ignition but I don't know if many people would consider it a problem. It's not like they're a consumable.
If you're talking about the inline 6 - 135, that goes back much further. They haven't made those since the early to mid 70s. I'd probably stay away. I've got a couple of dead inlines. They weren't that bad but not that great and I'm not a big fan of them.