Previous employer asked me if I wanted a laptop, but I prefer a large display, keyboard, etc. with a big fast machine and lots of working memory. Ergonomics are important to me, I have friends with permanent damage from poor posture. To see people hunched over laptops makes me cringe.
Keyboard and Mouse, obviously. Best to have them at an usb hub so you only need to plug in one cable (or a docking station).
Fast is relative. There are laptops where you can put in 6 core Xeons, but they are rather expensive. A modern mobile i7 quadcore or equivalent AMD CPU is rather fast. Try it.
On a powerful laptop you can easily have 32 Gigabyte Ram. If you need more, then you need a highend workstation, yes.
Poor posture doesn't need to concern you more than on a desktop PC if you have your laptop connected to the same peripherals than you would have a desktop PC connected to.
Laptops additionally allow you to take them with you and work with bad posture.
When you unplug you lose all that equivalent experience, and then you're on battery power... which sucks for powerful laptops. Working from a couch or whatever just seems ineffectual to my workflow. I can see media consumption working in that case, (not the latest games of course) but then you don't need the big beefy machine.
I really don't see the benefit of portability for a workstation. It's a luxury to me when I have less things to haul around. For work everything is either in off site backup or version control systems. Obviously that wouldn't work for something like video editing.. but then (and I'm not an expert) you'd want a bigass power hungry workstation anyway.
Regarding apple notebooks: Bluetooth keyboards are pretty much equivalent to Macbook keys, so most of my co-workers would just use that. The apple mice suck, so most would again use the notebook trackpad. External screens were used as expanded desktops with most work happening on the default notebook screen. Everyone was hunched over the laptop like it wasn't docked at all. This is my own limited experience, but it seems that in practical use, the limits of laptops somewhat encourage poor ergonomics.
The display adapters should be only a few dollars, so maybe not ideal, but hardly a really good reason.
There's not really a reason why a powerful laptop should be really bad on battery power. Modern CPUs save power pretty well, especially mobile ones. And any powerful GPU should have the capability to be disabled.
But still, it's mobile or "mobile". As long as it fits in a backpack and I don't have to hike for hours I really don't see the problem. Maybe laptops today are not completely feasible, but I suspect in one or two generations the successor to USB3/4/? will be capable of driving everything from screens to external GPUs or CPUs so you'd only connect one cable and have everything attached there. But that's not now. For me it's just a minor inconvenience for the big convenience of having everything with me. And everything is not only stuff I have synced or in a version control system (by the way, don't you have that moments where you are somewhere and remember that the code you want to access is on a machine dozens of kilometers away and you forgot to push it?), but really everything and everything exactly how I left it. It's just the feeling that I can just suspend and go somewhere, no matter where, get my laptop out and continue exactly where I left off.
"Laptop" does not necessarily imply "ergonomic disaster".
In a work situation, everything I do is in version control, or some external backup. So a notebook doesn't have any special data portability convenience. A dedicated machine at home and office just makes more sense for my purposes.