It is necessary to fit very well the accorded conditions. In our case, the initial conditions changed, in service support, licensing, etc. Some modules changed and it has been necessary to pay again for a functionality that we already had. The commercial section did not treat us well. At least in Spain, it is a risk not to have different providers that implement the solution; we are tied to only one provider (the same that develops the product). When the relationship with them is not good, there aren't other alternatives. Changing the solution is very expensive, in money and time, but we always have it in mind.
