I think the OP was asking about finding a job as an attending after already completing a fellowship. But yes, H/O has gotten to be almost as competitive as cardiology in recent years--about 20% of applicants this year failed to match.
Honestly, even once you complete a fellowship, my understanding is that the job market is pretty competitive. As an example, at one of my interviews this year, a graduating fellow said that there was literally one job opening in the entire state of Florida (which is where she was aiming to return), and they interviewed 2 from a applicant pool of >50. Another place I went, one of the graduating fellows was going to have to go back to general pediatrics because they could not find a job in the major city that they were geographically tied to.
I'm not saying you should look for another specialty--you should absolutely do what you are passionate about! But if you decide to do heme/onc, be aware that you probably need to be geographically flexible, both in terms of fellowship and final practice location. I will also echo what Stich said about becoming involved in research as soon as you are sure heme/onc is what you want to do with your life, as not only will it make you a more competitive fellowship applicant, but a longer, more productive history of research will also make you more competitive for sources of funding when you are applying for grants to fund your first attending gig.