The warranty question forced you to acknowledge that the tuners don't guarantee their work. They have no idea what changes they are effecting. They make good guesses, and send you on your way. Hence, no guarantee.
SVO manufactures who DO offer warranties:
1. Kit maker Elsbett GMBH, Thalmassing Germany
2. John Deere, USA
3. Audi , Germany ( a single tank kit for their new TDI )