Would you use MongoDB in high-transaction environment?
MongoDB can survive high concurrency writes, so long as you engineer for it.


INSERTs are about the same size.
Very little UPDATE going on.
Shard to add write concurrency scalability.


The things that will kill you on write scalability will be updates that go past the built in padding factor, as that triggers a double write as the block is relocated to a larger free spot. If your document sizes are within a narrow range and updates don't change the size much, it can scale quite far.