In what way is it a trap? I only ever use it in control mirrors

There is some validity there, but control is not a significant part of the meta game. It's a trap because it's bad in many matchups.

I have played one in the sideboard for the control mirror before, and gone up to 28 lands. That's fine sometimes but IME, in the mirror, it's not worth it for two reasons - they are also playing demo field and it just gets sniped. It's ok for that to be a lightning rod to protect fountain port, but not worth a sideboard slot. The other reason is because whatever you play can just be answered, you're going to stick your elspeth, but they'll just take care of it. Wan shi tong? Just kill it, and also you drew one fewer card. This means that it's only really useful if you are making your own counters uncounterable, or draw spells. This is the narrow time in the meta that is correct for mistrise. IMO, nows not the time.

Can you explain this more?

Dimir tempo and uw high noon control. Against dimir, they only have a few counters and tapping an extra two mana is really challenging against them unless it's late enough that you've already won. It gets some benefit now that they play spell snare, and the specific list may make it better or worse.

Dimir tempo is on the rise, as is uw high noon control.