There can be a conflict of interests between client and spreadbet provider if the spreadbet provider doesn't perfectly hedge the bet immediately in the market. The result of this is that the spreadbet provider can try to get an advantage by:
1) triggering stops
2) offering poor prices
3) refusing a price to open or close a trade (requote)
However, with all that said, as long as you know your enemy, you can get around these caveats. One way is to trade more long-term as opposed to trading for a few minutes at a time.
Edit:
I should also point out that only Direct Market Access CFD trades are true CFDs. The other kind are market made CFD trades, which are akin to spreadbets (without the tax break).