There's a lot more to the differences in the RR Trent family than simply "scaling up" - there are significant design differences between them in order to suit their application better. The GP7000 series (the alternative A380 engine) was not a from-scratch design, it was developed from the GE90 and PW4000.
I'm prepared to bet that when the A380-800 series lands for the last time, there won't be a significant difference in reliability or incident between the engine types.
Engines let go sometimes, always have. Because of this, designs are refined during the time they are made. For example, the original RB211 was considered something of a nightmare in terms of reliability initially, but by the time the -535 variant was installed on the B757, they had achieved a level of reliability so good that even the Americans had to take notice.