Yep. I'm wary of anything with not covered by the Financial Services Compensation scheme. I was nearly burnt by Icesave's collapse years bank so am a bit more careful now.
I use Revolut regularly but only top it up the card as and when I need it. I certainly wouldn't consider paying in a salary or using it to transfer large sums.
While I'm sure their structure and ring fencing of funds mean it's unlikely anything bad would happen, to me its not worth the risk. Once they have a full bank licence and have coverage from the FSCS I might change my mind. Tell then I treat it as a prepaid payment card that offers good exchange rates and the ability to access money overseas easily.