The spleen is an organ made up primarily of blood vessels. This makes diagnostics such as aspiration/biopsy/partial removal/etc difficult. Also when cancer is of a concern, the high blood flow makes for a large concern about metastasis (spread to other parts of the body). Therefore if a splenectomy is required, it should be done without delay. The spleen is easy to rupture which will lead to an immediate life threatening situation.
How do they do with the surgery? Even though the surgery is longer than other intra-abdominal surgeries; the vast majority do very well. The risk of problems associated directly with the surgery is low unless the spleen has already ruptured (which if it was at the time of ultrasound would have been detected and immediate surgery would have been recommended). The risk has more to do with what is going on inside of the spleen and its interaction with the patient.
How do they do postop? Not factoring in whatever may or may not be going on with the spleen, splenectomy patients do rather well. They immune system is slightly compromised due to this and any infection concerns down the road need to be dealt with more aggressively. Their life expectancy, again not looking at what may or may not be going on inside the spleen, should be roughly similar to a normal cat.