Also, I imagine they have little interest in complying with US LEO requests.
The NSA has greater authority (and likely, capability) to collect every single bit in a Yandex server than it has in a Google one. And, of course, so does the FSB, the security service of something fairly close to an actual mafia-run state. So that one doesn't sound super-convincing. What are some others?
Edit: you added a bit of detail about the IRS after I replied, are you saying a yandex email makes it easier to evade taxes? That the advantage of such an email address is skirting or breaking laws and regulations?
Here's a real example. Aluminum extrudes are highly import taxed UNLESS they are are an integral product feature. Does the IRS get to see my emails on that, pre-emptively, before any presumption of doubt? Should they be able to poke a few search terms into an NSA database? Or is the burden on them to get a subpoena?
I see no weirdness that they can see my books but have no business in my conversations. Decades ago, they couldn't peer into meetings in a real-time, prior reasonable doubt way. 1984 and all.
The iron law of "You Are Going To Be Mossad'd Upon" applies.
Presence of laws is compensated by absence of necessity to follow them.
No, I wouldn't trust anything stored there without encryption from my side.