Long vertical venting

Some brands claim up to 100 feet of vertical vent pipe while others are as little as 35 feet. Are the tall claims real-life or pie-in-the-sky?
I'm considering a condensing tankless in the basement of a single-story home with an unused chimney flue. Presently about 25 feet but that could increase if I build an attached garage.
I would line the flue with the 2 3" pipes--if they'll fit, including the elbows. I might have to get creative if they won't fit.

To get certified, it must work as listed in the installation manual...so, as long as you use the proper materials, within the specified lengths, you should be okay.

Keep in mind though that length is the 'effective' length, and elbows shorten it by a fair amount, so your actual length may not approach that maximum actual length depending on the number of elbows in the layout.

I've seen small metal vent caps poking out of a chimney flue before--but can't recall ever seeing pvc sticking up there. Is it common to vent that way? I'd rather go through the wall but the wall above the basement utility room doesn't have adequate space around windows to locate the vents.

If that device is the only thing utilizing the flue, it shouldn't matter...the pvc cannot be surrounded by the flue gases of anything else that might use the chimney...the max rating on the stuff is usually around 140-degrees F and it starts to soften and distort when you get above that. You would need to support it so it doesn't flop around, and that may only be possible at the top and bottom...plus, the stuff does expand and contract quite a bit in length (and girth, but it's the length that can be an issue). This can lead to stress and popping noises if it is constrained improperly without that consideration.