What should contractors compare when choosing an umbrella company - apart from the weekly margin?

Contractors often compare umbrella companies primarily on the advertised weekly or monthly margin.

But that surely shouldn't be the only consideration.

What else should a contractor genuinely check before choosing an umbrella company?

For example:

transparency of the payslip;

pension arrangements;

holiday pay treatment;

insurance;

salary sacrifice;

accreditation;

customer support;

how deductions are explained;

payment timing;

compliance history.

I’d particularly welcome input from umbrella specialists and accountants here — but please keep responses educational rather than promotional.

Contractors: what have you personally learned from using umbrella companies, good or bad?