Should I take a Data/ML job at a startup that doesn't yet have data

After 7 years of frontend web app experience, I'm switching careers to ML/DataScience and looking for that first job to get my foot in the door. I recently spoke to a founder of a very early-stage startup. (I don't have any experience working with startups either.) They seem to have their frontend work outsourced to a team in another country but their core product will eventually depend entirely on a recommendation engine that will need to be built by new people. If I take the job, I'll be the first Data person, but also the first in-house tech guy so I might end up with leadership responsibilites later on.

My question is whether it makes sense at all to join as a "Data person" when the data will likely be insufficient for the next 12+ months. They won't even launch the app until January.

I worry that I will get caught up in mostly doing urgent frontend work and putting out fires and it will be at least a year before I even begin to look at the data.

Thanks for any advice and for pointing out my naivete!