The simple fact is, SIMS is crap, but so are most of the others.
We know this, they know this, but you're locked in, because changing MIS is an absolute ball ache.
So much of the school operation is tied to your MIS - admissions, registration, timetable, behaviour management, HR, payroll (for some), even document storage - and you can't just switch package without a huge investment in time and money for training new staff.