Changing the rest structure will impact the deadliness, especially if they're forced to push on after most resources have been spent. The 8 hour short rest and 1 week long rest makes even simple tasks much harder.
Change Death by Massive Damage to be extra HP equal to Level plus Con score. At lower levels, it's not likely to matter, but soon healing from 0 HP becomes a risky process. With only a few HP, taking lethal damage is much more likely, even without a critical hit. Being attacked while down is much more likely to kill, since it's also a critical hit, and random AoE can also cause instant death (rather than just a failed death save).
I will quibble with the first statement.
you can inflict time pressure to keep going with a 1 hr short and 8 hour long with no problem. Ongoing harrassment by enemies, chase and pursuit angles and even just the usual sacrifice at midnight tic tocs can push those just fine.
So, no, changing the rests to longer dont make things deadlier than they can be under the normal rules - they just expand the number of times and ways you can work in those pressures from their "lots of ways" to "more lots of ways" *and* add in a lot of downtime to deal with (for better or ill.)
Folks always seem to assume its deadlier because "the old approach wont work" but what will happen is **unless the Gm/situation puts time pressure and/or interruptions in** they just change the frequency of their rest vs action vs downtime.