I can buy tickets for the same united flights from either United or Lufthansa. The lufthansa flights are listed with Lufthansa flight numbers, "operated by United Airlines." The tickets purchased from Lufthansa are considerably less expensive.

Is there any downside to buying the tickets from Lufthansa?

---Do I get the same access to E+?
---Do I lose anything in terms of mileage credit?
---Do I lose anything in terms of EQMs, lifetime miles, or 500 miler credits?

If there is an irregularity, will it be united's or Lufthansa's responsibility to rebook me?