Other sources have done their estimates from time to time. US, british and similar countries have talked every few months (they also give ukrain numbers) and while nobody agrees all are close enough to conclude ukraine numbers are trustworthy.

It sort of does go both ways and you should always take these numbers with a grain of salt but the Ukrainian numbers are at least possible and probably somewhere in the right ball park whereas Russia keeps claiming to have destroyed more of various western systems than have ever been in Ukraine for example.

Because to the extend it's possible, the numbers from Ukraine are pretty much confirmed by estimates by Pentagon and British military.
Apart from that, Ukraine is NOT run like a giant propaganda machine, but for instance has free press, which Russia doesn't. And Ukraine acknowledges what's going on in the war, which Russia also doesn't, except when it suits their propaganda.

These numbers could be propaganda, but I think Ukraine wants us to know the actual truth in this regard, the truth is horrific enough so they don't have to make it worse. If Russia were to publish numbers that are true, they'd have to acknowledge that their losses are higher than Ukraine.
But in Russian news they claim the opposite. Just as they claim Ukraine attacked Russia first, And they even claim they are also attacked by NATO.

Have you ever seen a false claim from Zelenskyy regarding the war?
Have you ever seen Putin be truthful about the war?

It's very clear that one side is way more trustworthy than the other.