He was one of the first deputies out there and 
he's still doing it all by himself.  While 
practically all other deputies has a staff of 
some size, he doesn't.  Furthermore, he has a 
serious day job that sometimes even takes him 
away from home; the deputy service has always 
been and will always be a service he offered.

I don't know anything about your friends' 
situation but Sato is a guy who you'll need to 
build your standing with; he's not going to 
extend any special treatment with someone or a 
group of people whom he thinks can be unreliable 
(or whom he knows nearly nothing about).  He's 
also the type of guy with nearly zero tolerance 
for attitude; he doesn't want to bother with it 
and he won't mind if you take your business 
elsewhere in that case.

He also makes mistakes, yes.  He gets stressed 
out once in a while, true.  His patience wears 
thin sometimes, but heck you'll get that with 
plenty of other deputies as well (and they can 
be even worse).  Putting his day job ahead of 
the deputy service, he'll do it every time (even 
if it means slowing things down for weeks).

But on the other hand, those who deal with him 
consistently and faithfully also get a person 
who will ask seller questions for them (few 
other deputies ever bother), combine shipping of 
items from the same seller over an extended 
period of time, and do a few other services the 
other deputies don't provide.

So is he the most reliable?  Not really, so 
certain people may be better off with another 
deputy.  It's up to each person's choice.

In the end, he's just one guy who's doing this 
on the side and he's not going to change that.  
Anyone who deals with him eventually needs to 
accept those circumstances.