They ARE rubbish, but not for all the reasons given..
... they are not economic to service, and about as un-green as you can get. I have a Sony Vaio lying around, which was £1200 new, died after 18 months, examination revealed that board had died, so a perfectly good screen, the most expensive component, wasted and probably going to end up in a landfill one day.
Nearly all laptop ports are directly on the motherboard, one bad accidental cable yank, and you'll need a board change.
Desk top's? No component will usually (£400 graphics cards not withstanding) be worth more than 20% of the whole things, motherboard, CPU, etc. Anything dies, getting a replacement is easy.
My HP Envy 17's Blu-ray drive died, they fixed it under warranty, but if it was not, it looks a total pig to open even for that to be replaced.
Oh and my fantastic Acer's. One was lasted the distance, despite hinges snapping, which I was lucky to be able to get second had, Acer spent a year trying to get me the right ones, and eventually said it was too old for them to get parts for. Now however the machine has a great display, and Windows 7 works quite well on it, but driver support has gone, none for the TV card, and waking Windows results in screen gunk, fixable by doing a sleep and wake again.
The Acer 8920G, those Gem Blue ones, well its sound has 'gone', and yes, it's a chip on the motherboard, surface mounted, so we used external USB speakers with an audio device built in. Oh but then its inverter died, so no screen either, time to use with an external monitor. Yes I bought an inverter for it, but for some odd reason they don't last long. Again most of machine is fine, let down by small issues, rendering machine unusable.
Driver updates from manufacturer? Typically years behind desktops, if they happen at all in some cases.
They are also 'quirky', 'something' about their architecture will usually result them being slower than a desktop with the same spec. The Envy 17 exceeds (i7, vs i5) my desktop in performance on paper, bar the graphics card, but still feels slower despite a fresh SSD and a clean Windows install, life's too short to work out why. Maybe there are Fn key, power and screen management etc. issues, i.e. laptop related hardware.