It makes 'em unreliable on the road.
I gig a lot so I'm kinda like a worst-case scenario for an amp.
I
don't bang them around ... but they vibrate down the road a lotta miles in my truck and then get rolled along on rough pavement or even cobblestones on my dolly ..... shoved up and off stages.
Carried up and down stairs.
All with the inevitable bumps and bangs into doors and such.
Surface mount components are a PIA.
They're why my Marshall 6101 won't work for more than a few weeks ....
They're why my Hot Rod deVille (which I like a lot) sits on the floor .... I'm tired of having to drop all the knobs so I can pull the pre-amp board to get at the solder joints that fracture on those surface-mount pots.
They're why I don't use my Ampeg ReverbRocket re-issue anymore ....
That's why something like a Mesa (or presumably the better Marshalls) have chassis mounted pots with flying leads.
When you have a circuit board with pots or jacks on it .... the weight of that pot or circuit board vibrates and has enough leverage to fracture the solder joints and you start getting intermittent outages or staticky sounds or whatever. Then it'll work again ..... then go out again. PIA!
Then eventually the solder joint fails completely.
And jacks are really bad about it because you constantly plug and unplug the cord which flexes the jack every time.
I HATE surface mount jacks and pots