About Umbrellas
Reading the pages on this site may give you the impression that umbrella companies and agencies are all disreputable and not to be trusted. This is not the case, and in fact we are aware of a few who have been in business for many years, appear never to have put a foot wrong and are (probably) a completely safe way to get paid for work inside IR35.
BUT... For every good one, there's another that's either running on shaky financials, or is flat-out illegal. Until such time as HMRC clamp down on them, or they are regulated by the government, this will continue and inside-IR35 contractors need to be careful not to get caught out by them.
Umbrella Business Models
Some reputable umbrella companies simply work on the basis that they provide payment services, for which they take a fee. That is, the payer pays them money, they deduct tax and other deductions, take a small fee and pass the rest on to the worker. Their businesses are structured efficiently and they are cost conscious and so can operate this way and still make enough money to run their businesses.
However, some umbrella companies also provide "referral" services, particularly to recruitment agents, but possibly also hiring firms. That is, for every recommendation they get from a recruiter, they pay a fee. This gets the umbrella more workers to process, and so they can extract fees from more people (and the agency makes money too) - but paying all those referrals costs money, which usually results in higher fees being taken (which the contractor will pay).
In extreme cases, the umbrella may find that it cannot make enough money on fees alone (especially if they pay referral fees). Disreputable management in those organisations may resort to fraud, either on a time-limited and small scale or on a more overt, massive scale. Either can put contractors at risk.
Umbrella Referral Arrangements
You can ask anyone recommending an umbrella about any arrangements they have between them. They don't have to tell you, but most should be happy enough to admit they get/give a referral fee.
Whilst a company may have a referral deal with an umbrella (which binds them contractually), this contract is between those two businesses and in no way benefits the contractor. That is, should you, as the contractor, need some help which requires co-operation between these two companies, their referral deal doesn't oblige them to help you in any way - if they do help you, it's at their discretion.