The distributions are sustainable yes, primarily due to financial engineering.
I am not a fan of split corporation funds. Many people buy them, enticed by the high yield before realizing what they are. Here is a generic explanation:
A $100 stock yields $5 a year. A split corp "splits" this stock into 2 worth $50. One carries the $5 yield. This is the preferred split share. So, you've doubled the yield.
The other is called the capital split share, this is what retail investors buy mostly. It does not have any exposure to that dividend. But, it is also highly leveraged to the stock price. Why? Because preferreds are guaranteed their money back.
If you buy the split preferred, you pay $50, get the $5 a year dividend and you are guaranteed your $50 back. The sacrifice to this is you do not get exposure to the share price movement. Capital shareholders do. In fact, they're leveraged to it. A $50 capital share is exposed to the movements of a $100 stock. So if that $100 stock moves up $10, it has gained 10%. But you, as a capital shareholder of a split corp, have saw your price go from $50 to $60, or a gain of 20%.
That being said, if that capital share drops to.... $80, you've now saw your $50 shrink to $30.
Many funds are structured in different ways. But this should give you a general idea of how a split share system works.
But you can see how a crash would significantly impact your share price. Say you buy the $100 stock, the market crashes and it falls $35. You're now left with $65. If you owned the capital shares, you had $50 and it's now worth $15. This is why when you look at some of these split fund corporations during crashes, their prices absolutely collapse, sometimes 65-70%.
If you need income, they provide a solid option. And for lots of people who bought them in the midst of the pandemic, they've realized some strong capital appreciation. However, they shouldn't get used to this. These funds have a strong history of depleting the NAV (net asset value) to pay the distribution. It's exactly why if you look to the stock charts of many of these funds, they're in a consistent downward trend.