EPDM Rubber Bond Roof in multple pieces
It's perfectly possible to have a lap seam in an EPDM rubber roof.  

The important thing is that the lap seam is horizontal, with the upper piece overlapping on top of the lower piece.  

The strength in a rubber roof is in 


the fact that no one has destroyed the rubber chemically by attempting a repair with random cheap roof-patch materials from the building supply, which invariably contain tar or other chemicals incompatible with rubber roofs
how well it's glued down to the substrate
how well the seams are glued. 


All of these can be well done, to answer your question.  (But the first one is up to you.)