For my part, I do allow long rests in dungeons and other adventure sites, as well as on the road. However, taking a long rest in a dungeon is extremely risky. I roll for complications every hour in adventure sites, plus additional rolls when the PCs attract attention to themselves. Taking an eight hour rest therefore carries an extremely high risk of interruption, which may result in being unable to finish the rest and receive its benefits. As well, the dungeon’s inhabitants are likely to take precautions against further assault by the PCs unless they are unaware of the PCs’ presence. But personally, I prefer to leave the option open to the players to take that risk, rather than outright telling them it’s impossible. Weighing risks and making those tough decisions is one of the key selling points of D&D for me.
During travel, it’s a bit of a different story. In travel mode, I roll that for complications once a day, and additional times when the PCs take risky actions like traveling through particularly dangerous territory, traveling at night, or resting without a watch posted. Accordingly, I end up rolling 1-3 times most travel days, and about twice as many times on days when the players are forced to trek across dangerous territory. Long rests are an expected part of each travel day, and assuming the PCs take their rests at night, it is the time complications are most likely to occur. So, again, there is a high risk of interruptions preventing the PCs from finishing their rest, but in that case the alternative is taking a forced march or going without rest, either of which risk gaining exhaustion levels. The risk of interruption can be minimized by resting during the day, but this will mean a greater risk of more encounters during the adventuring day, since traveling at night is more dangerous and triggers additional rolls for complications.
