Craig, these are some excellent points. With matched betting we always use the exchange to lay the bet that you place at the bookmaker. So you are only placing 2 bets instead of betting each leg at a different bookmaker. This has a couple of advantages. firstly you are betting normal round stakes at the bookmaker which looks far more natural, if you are trying to cover each outcome at bookies you'll end up placing bets like £6.67 on a draw, which is a red flag and will likely lead to accounts getting closed. Also by backing at a bookmaker and laying at an exchange you are betting on the same outcome, both bets just mirror each other, so there it's highly unlikely that a leg will get cancelled from a mis price.
Also with matched betting you are using the bonus cash to create artificial arb opportunities in effect, so you can make profit on less suspect or mispriced markets, for example premier league games. It's essentially a way of extracting bonus money risk free.
With £50 - £100 your returns won't be as high as they could be, and you will need to wait a bit to withdraw funds from bookmakers to top up your exchange, but plenty of people have turned that £100 into £1000 within a month. What tends to happen is people see that it works and then invest more into their exchange account to speed the process up and make larger profits, but that's purely down to the individual.
You are absolutely correct that bookies can go bust. We actually have a section on Yesbets that shows bookmakers that we don't recommend, usually because they have no UK license or little is known about them. All the offers on Yesbets are from large UK bookmakers to minimize this risk. It's something that we take incredibly seriously and is a vital part of the service that we offer (keeping you and your money safe).