I/O impact does not really matter of business size, it is always 2x for writes
and unless you are talking about SSD storage, then I would be really concerned about that. When planning storage for any virtual environment, you will find that IOPS is almost always your primary constraint. You will often need to go with twice the capacity you need, because only that amount of spindles will provide the required level of IOPS... so thinking that IOPS will never be a problem is usually wrong. They will become the problem at the worst possible time, when it is too late to do anything.
Overall, there are plenty of other differences, here is my top 10:
1. Hyper-V replica doubles write I/O on each protected VM
2. Veeam supports file and item level recoveries from replicas.
3. Veeam is not limited to 24 restore points.
4. Restore points are stored compressed with Veeam.
5. Bandwidth consumption considerations:
a) Hyper-V does not support page file exclusion.
b) Hyper-V replica does not support traffic throttling
c) Hyper-V replica is very inefficient due to journaling approach, when Veeam needs to transfer only latest state of changed blocks.
6. Cumbersome seeding requires VM cloning, with Veeam you can just seed from backup (including VeeamZIP).
7. No flexible scheduling (limited to selected intervals)
8. Data processing (such as traffic compression) uses significant host resource, when with Veeam you can offload this processing to off-host proxy.
9. No WAN optimizations (Veeam provides TCP/IP multithreading, optimizations for high-latency links).
10. If you go with Hyper-V replica, you still need to buy backup software! Replicas cannot provide neither required retention, nor meet the requirements of having at least 2 copies of your production data. And if you buy Veeam for backup, replication is included at no extra charge as well, so you don't even need to make this choice in the first place...