Insignificant?
It would be significant if you put enough money into it.
If I'm making many loans of say $100 each it's a lot of work to either write off the losses or to compute capital gains and losses on traded notes. It's not that the returns are insignificant in aggregate, but the amount of work required for $10k invested in LC vs. a junk bond is significant.
It could be mitigated by taking larger positions in each loan. But I'd rather not hold $10k worth of debt to a single irresponsible borrower refinancing his credit card debt.
FWIW, I did put $6k into Peer Street. The minimum loan size is $1k. And they're all interest only loans with a final balloon payment. This means there won't be any difficulty figuring out any losses at tax time. Again, dealing with this on LC was labor intensive. Though perhaps they've improved their reporting tools in the year since I've been an active lender?