There is code about the length of vent and number of 90 bends. I could not imagine climate not being relative to condensation and pipe angle either as it related to rotting out walls. Mostly Its fire related. I would reflect upon what you google since new experience with fires may have changed it from what I was using last, and your post does not really get into details like current length of run or angles or type of existing piping.

We have a dryer in the garage, and the length for the vent is huge. I am surprised that it was done this way. But to be honest, if works just fine.

I have used ventless dryers when we traveled in Asia, and home-owners without fail placed them outdoors (often in a closet on the balcony). I suspect that this has something to do with the fact that they do in fact heat up the place noticeably. It's simple thermodynamics, any heat that is produced eventually has to go somewhere. Heat pumps and insulation can only do so much trying to contain it.

I lived overseas for five years and had a ventless Bosch dryer. I thought I would hate it and would always be forgetting to empty the tank. I never did and actually did not mind as the sink was right next to it. It became a habit just like removing the lint before the next load. If I had a small space, I would use a ventless again. I also read that the water that comes out is distilled (not sure this is really true) and could be used for other purposes.

A long run with lots of bends would be a fire hazard in my opinion.

I once had a long duct with a 90 degree bend. I had the typical dryer doesn't dry problem as a result of a big lint clog in the duct. It was hard to get it out but we somehow finally managed to get it through to the outside and fished it all out though the louvered duct cover. Dryer worked great after that.

I bought a new FL washer/dryer last year and needed a set that wasn't too deep for my space. I just couldn't get my head around the idea of ventless which is what several of the smaller models had. I opted for something else so no experience with ventless.

I have a ventless Bosch because I didn't want to make a big hole in the wall of my stone house. I love it. Also had one in a Manhattan apartment and it was fine. Yes it does add some warmth and humidity to the bathroom it's in, but great for a hanging fern!

How long is very long?

We have a ventless Blomberg heat pump dryer in a smallish basement bathroom and I haven't noticed any particular heat or humidity issue. The cycle takes longer than a vented dryer, which I don't find to be a big deal.

Humidity shouldn't be a problem with a condensing dryer, since the water from the clothes, by definition, condenses and drains into a tank or down the same drain as the washer water (ours does the latter, so no tank to empty). Also, I suspect that heat is less of an issue just because of the way the technology works. As far as I understand, a vented dryer adds heat through a gas or electric heating element; a heat pump dryer, on the other hand, just moves heat from one part of a coil to another, with the hot part heating air in the drum which is then blown over the cold coils to condense out the water. Since heat isn't actually being added, the hot and cold parts of the cycle should largely be a wash (ha, laundry humor).

I'm not sure how condensing dryers work if they AREN'T the heat pump variety, so I can't opine on heat from that kind of a dryer.