The 840s (and 800s through 820s, I believe, although I haven't had to service those in a while) have every major component partitioned and blocked by a plastic sheath. They're tool-less, but the hook and catch mechanism (especially on the expansion slot access) plus their flimsy structure makes them tiresome to constantly remove and replace, and difficult until you've done it many times to know when they are properly seated—for a while I wasn't putting it in properly and didn't realize until I tried to replace the main door and it wouldn't latch. At that point I'd prefer just a cowl like the G5s had.
HP advertises their Thunderbolt 2 support, but installing the cards are a pain (have to remove everything you have in the system to run little wires and switch jumpers on the mobo) and their documentation was as of May 2015 when I had to do the installs, incredibly poor (the cards shipped with incorrect install guides... finding the correct ones was an exercise in frustration, as their support site is even worse than Apple's.)
Then there's minor things, like how their PCIe slots are positioned at the very bottom of the machine, making swapping video cables more difficult. For my use case, the 1125W chassis for beefier graphics and dual processors comes with 3 six-pin connectors, meaning I still needed a cluster of harder-to-find adapters to power a Titan (and that extra cabling interferes with the sheaths much more than the stuff I've done in a cMP to get extra power.)
I appreciate that they are more quiet than a cMP and fairly close to a nMP's noise levels, and despite being a lot bulkier and heavier than either are generally a bit more wieldy than the cheesegraters (for the cMP, though, I always picked them up from the underside, so the cut hands was never my issue.) They certainly can be kitted out with a lot of power, although in my use case that's largely going to waste (not HP's fault, though, that's Adobe's.) There's things I like about them, and things I don't; every tower design has its drawbacks (as fond as I am for the G4s, with their molded handholds and their accordion design, getting into the hard drive cages is still incredibly frustrating). Just like every pro has different needs from their computer and issues that are deal breakers to one person on this forum are irrelevant to another, so to are designs. It's not a matter of quality, it's a matter of priorities.