Yeah I believe the long term risks are very low, probably only worth considering if you have issues and are on other meds it may interfere with. There is research which shows that Natto can dislodge clots which is problematic if you have any.
Guess the main consideration is the cost and efficacy, especially when the trials which i've read appear to have dosages from anything between 2000FU-14000FUs a day. If your body actually needs the upper end of those doses to have efficacy it works out to be very expensive. I think it worked out at nearly $50 a month for me to take Natto at 14k a day.
Be careful when buying as it appears lots of companies just put the mg unit down per tablet, not actually how many fibrin units there are.
I looked into it quite a bit as have borderline high blood pressure, went with Telmisartan in the end, lowers blood pressure, reduces visceral fat, mitigates LVH.