I've never used an arb service. I place arbs against BF for approx £100 - £200, mainly to keep my points balance for the commission discount. 2 things stop me getting deeply into arbing.
1) Account limits. Most UK bookies will limit you pretty quickly if you start betting large amounts on out of line odds. They don't always tell you so the first you know maybe when they refuse your bet. Imagine you've placed a bet for £1000 on one side of the bet and your limited to £20 on the other side
UK bookies that have limited at least one of my accounts so far are Ladbrokes, Paddy Power, Stan James, Boylesports, Betfred, Bet365, William Hill, Betpack & Totesport. None of them contacted me to tell me I was limited, I found out the hard way.
In order for an arb service to remain profitable you need as many bookie accounts as possible.
2) Palpable errors. A bookie may refuse to pay out a bet and claim it is due to an error on their website. This is usually due to an error in the odds offered and is usually when there is a gross error, such as offering 20/1 when the average at other books is 2/1 etc. However the provision doesn't stipulate what constitutes a palpable error so there is in theory no limit to what a bookie may palp. Enforcing that decision if you appeal is an entirely different matter and so long as the odds aren't way out of line it would be unlikely that you would lose this way.
There is another type of palpable error too which is if the bookmaker has the bet listed incorrectly, so for instance wrong venue, wrong start time etc. Under these circumstances virtually all bookies will void the bet and return your stake.