They are great, I've had one installed for 8 years now with no issues so far. I cleaned the sections of both pipes with alcohol before installing, then laid down a bead of silicone and installed the fernco on top of that. Its recommended practice for joining to cast iron since it is so rough on the surface, might not be a bad idea for you.
Assuming you somehow use up your original data cap in... Let's say one incredibly data intensive day somehow. That leaves you with (30 days * 24 hrs/day) * 3600 seconds/hr * 256,000 bps = roughly 660 gigabytes. So I guess that's probably the limit? Plus your original cap.
They are great, I've had one installed for 8 years now with no issues so far. I cleaned the sections of both pipes with alcohol before installing, then laid down a bead of silicone and installed the fernco on top of that. Its recommended practice for joining to cast iron since it is so rough on the surface, might not be a bad idea for you.