Yes it is viable but uncommon. As with everything to do with investment, you have to know what you are doing and must have a plan.
I have been successful with long term trading of CFDs for several years. It is true that the cost of financing to hold positions long term cuts into profits but so do the spreads and comissions when you trade frequently. You need to use the benefits of CFDs to your advantage: variety of instruments, being able to hold long or short positions and leverage.
What I have found works is a portfolio that is has overall low volatility even if its components are significantly leveraged and/or volatile. I backtest portfolios with my main focus being the maximum drawdown I can safely accept. Provided you can generate a return on your investment that exceeds the financing cost and can manage drawdowns without a margin call, there's no reason this can't work for you.
Note: I've found it critical to set up stop losses and stop endries (especially on volatile short positions) to ensure my portfolio doesn't drift.
There are additional advantages of this strategy from a tax perspective. In my case, holding CFD contracts long term are unrealised capital gains so they aren't taxed. However, the financing costs are realised losses so are tax deductable. Of course it may be different in your jurisdiction so check with a tax accountant.