I never said I was against your argument, I was just pointing out the mistake. I can pretty easily see both sides here. Reimu is definitely shown to get tired when using stronger abilities, so I could see that activating Fantasy Nature and using it would put her under stress. However, Marisa's description that she just closes her eyes and everything happens seemingly automatically is also somewhat convincing that it's effortless; at least more convincing than ZUN's spell card quote, which as mentioned, is often flavour-tinged and prone to exaggeration.
I don't think it would be prudent to assume that she does or doesn't get tired. Using either of the current arguments to pick either way is too much to me, because I don't find either side convincing enough on their own, never mind when considering they should outweigh or explain the other.
Also Eirin's Last Word doesn't say it's undodgeable either, it says "once she spots you, escaping is no longer possible", which is a play on 監視から逃れる, meaning to "escape someone's attention". So, "once you've caught her attention, you can't get rid of her". Very Eirin.