Look at it another way -
If you pay a commission of £27 to pay, let’s say a third less tax, although probably a lot more than that…
If your weekly wage is say, £500 and your deductions are reduced from £150 to £100 thanks to the umbrella scheme, then seemingly everyone is happy. You take home £23 extra, the umbrella firm gets their £27 commission…
HOWEVER - it’s only possible to reduce the tax payable in this manner - by maximising expenses. This is VERY difficult to do if everything has to be “receipted” as I’ve found… I have been disallowed “Meal” expenses, because I can’t prove I always have a meal, despite the fact that I obviously eat and drink every day to survive. Tax relief on the cost of a meal doesn’t pay for that meal, so I eat at home or take a packed lunch - which of course I get told “disqualifies” me from claiming for something I need to do every day. I’m not Ghandi on a “holy fast” FFS! 
Mileage expenses only works out as “tax relief” on 45p a mile - which is 9p per mile towards your commuting costs for basic rate taxpayers. Again, “not enough” compared to the REAL cost of commuting umpteen miles each way all the time.
Without being able to max out the expenses - you can’t make umbrella pay. Even top-up benefits are too easy to get disqualified from now, because too much of the earnings fail to be left off the top line - which of course is where and how Wayne Rooney gets most of his “savings” from.
Then there’s the hidden effect for the years to come… Having paid less deductions out over the years - your state pension may well be knocked down.
Then what about the so-called latest “scam” of “compulsary opted-in pensions” where your payroll firm starts deducted pension contributions, non refundable, totally unsuitable for zero hours employment, and when you try and “opt out”? - You get ignored, and the continued theft of your wages continues…
I’m sick of it all, and I’m going back to full time where I’m actually employed properly at the earliest opportunity. 
The last straw was when I got “backcharged” expenses - for attempting to get my full annual mileage allowance more towards 10,000 miles.
Obviously, Umbrella outfits are allowed to play games with the tax rules - but ‘staff’ are not. 