I would say that saying 'almost all of them will basically work' is a bit
extreme. People often have an incorrect assessment of the situation because
they aren't buying new laptops at retail frequently. Most people are
installing on older hardware which is less likely (although still very often
problematic) to be a problem. NONE of the newer Intel systems for example are
a good choice for free software users that are sold in store.
That said you'll be more likely to be able to get everything work by buying
an older system with Intel graphics only that isn't Dell, HP, Lenovo/IBM,
Toshiba, Apple, or Sony. The later companies use digital restrictions to
prevent users from replacing the wifi chip with one that works. That doesn't
mean you won't run into problems with other components. For example many
laptops have on/off wifi switches which are off-by-default. These systems
won't work or won't work well because you can easily (even if they work at
first) accidentally turn off the wifi and not be able to turn it back on or
it might not work at all as it is default'd to off. This is even when you
have a compatible wifi chip.
There are other problems with keyboards, screens (ie no backlight? the guy
reverse engineering the Intel graphics chips isn't doing it any more in
protest of Intel's half support for GNU/Linux, this despite it being better
than others like NVIDIA/AMD), touchpads, and similar components.
It's far more difficult to buy a random computer and get it working right
than most people seem to think. Even when stuff does work it often fails at a
later point for one reason or another. There are very very few systems out
there that are going to work really well over the long haul.
