In a report released earlier this year, the FCA found "several causes for concern" about robo-advice, including concerns that clients were not receiving suitable advice.
Nutmeg, which is a robo-adviser with about 60,000 customers and assets of £1.5bn, reported losses of £12.3m, while Scalable Capital, which has assets of more than €1bn (£0.89bn), reported a loss of £1.3m in their latest accounts.
Meanwhile, Moneyfarm has posted losses of £13.9m despite seeing its assets under management grow by more than 50 per cent to £400m.
Simon Miller, who works at robo-advice firm Scalable Capital, said: "I think the point being made around customer acquisition is an old one which has been documented several times. Particularly at a time when certain robos, including ourselves, are starting to reach the scale and growth rate which will take us past profitability.
"The point around uncertainty and interest rates is of course true but it is true for all firms trying to raise capital as we move into a tighter monetary cycle.
"That said we are well funded and are confident that our business model and our growth so far put us in a strong position. I would also add that we do not rely on a standalone direct to consumer business for our growth. We have a number of business to business to consumer partnerships which are live and contributing significantly to our growth alongside our direct to consumer business."
Scalable Capital has a partnership with ING in Germany which generated €500m (£445m) in AUM the first 8 months, he said.
Toby Triebel, chief executive for Europe at Wealthsimple, said: "Customer acquisition is one of the challenges of not being a 200 year old institution that commands trust because it's big and has been around for a long time, but where new players are standing out is by delivering more affordable and accessible services that are driven by an exceptional customer experience.
"Given the nature of the service we provide, we're focused on building long term trust with our clients which makes growth channels like referrals an incredibly important part of our business model."
A representative of robo advice firm Wealthify said: "The claim that our business model is unsustainable, typically comes fro traditional services that feel most challenged by new providers like us. The truth is that the way consumers are demanding services, including financial services is changing.
"89 per cent of the UK own a smartphone and almost three-quarters of users have at least one financial app on their phone. This demand for digital solutions will only continue to increase, particularly among the younger generations, who will want to manage their banking, saving and investments online via easy to use, smart, intuitive apps."
They added: "The assumption around acquisition is that it’s predominantly expensive above the line activity, when in reality there’s many ways in which we engage with and attract customers. Our app is top-rated in the app store, we engage extensively online, through content and influencers and through our partnership with Aviva – these are just some of the ways we can start a conversation."