The SX1272 has very good rejection at >1MHz offset for an ISM band chip. ref datasheet. However, if you try to receive a weak signal close (<50m) to some GSM BTS you may encounter an interference blocking issue. We have seen a few situation like this. In that case the only solution is to insert a SAW filter (863 - 870 MHz) between your antenna and the chip input. The most common problem we encounter are poor RF matching on the end device module (typically hope RF modules have poor matching) , poor antenna radiation patern (no secret , a small antenna has a squashed spider like radiation pattern), and some time totally inapropriate gateway setup. The problem is that LoRa tends to work at a fair distance even when antennas are not connected and i have seen this several time already. You are right the XTAL offset and jitter are taken care of by on chip calibration and PLL.