As an ex IT engineer (mainly focusing on Apple kit) of some (deep breath) 10+ years, I have to say, I don't agree. Not when it comes to Mac Mini's.
I've swapped hard drives in macbooks, powerbooks, ibooks, imacs from 1999 to the present day, heck I even fixed up a mac cube. I've had 3 or 4 mac mini's and worked on loads...until 2011.
The most fiddly, irritatingly non user upgrade friendly mac I've ever worked on was a 2011 Mac Mini. Almost the same layout as the current one.
Other macs have been a pain, but you rarely felt like you were actively in danger of breaking it irreparably.
I'd never broken a mac until then. Not one. Sure I'd pinged off the odd aerial and needed to replace it etc.
But on this I cracked the motherboard - because there's a part - after you've carefully removed fiddly hardware which feels very nasty - you're basically prising things off a motherboard - where you (and I quote);
"..remove the logic board, using the two cylindrical rods of the Mac Mini Logic Board Removal Tool which must be inserted into the holes ... Inserting instruments into any logic board holes other than the ones highlighted in red may destroy the logic board."
Except you work that out by feel. Trial and error? Nope. Dead mac mini.
http://www.ifixit.com/Guide/Mac+Mini+Mid+2011+Hard+Drive+Replacement/6422
Step 15.
So I'm now waiting for the next mac mini to come out and buying it with a 1tb drive as I simply can't afford to irreparably break another Mac Mini 2 years on.
I'd normally applaud the comments of mvmanolov, I've done countless repairs and upgrades and it's amazingly simple in so many cases - but the mac mini is just too expensive to mess up.
It's absolutely not an upgrade for beginners. Memory is fine, hard drive, nope.