I think alot of times...most designers think in-house = not goot enuff...which i don't think is true.
Let's break into different parts:
(wall of text)

1. Deadline.
[ Agency ]
Most of the times, creative find themselves rushing deadlines after deadlines, long working hours
is pretty norm in ad agencies and if there is a pitch coming, you might kiss your wkends goodbye or
even public holidays.

Why deadline is so tight???
Coz the accounts hab to "communicate" properly with the client...and come up a brief..if the client
have disagreement about the approach/direction/strategy...the brief will takes abit longer and
when they finally agreed everything...the acct will brief the creatives.

At this stage..your process/working time is shortened to a few days...and you have multiple briefs to juggle.
Yet ideas cannot be compromised and thatz where your long hours come in.
After you & ur writer thought a wonderful, award-winning idea....you present to your CD and thatz where the fun begins....
CD approve > proceed to Accts approval else back drawing board.
Accts approve > proceed to client presentation else back to discussion wif CD again.
Client approve > pending for client's boss approval else back to rethink strategy again.
Client's boss approve > idea sold!

So you see....you have 4 obstacles to tackle...b4 ur idea is born.
so be glad if you have 1 shot wonder....clear all obstacles without any glitch.

And don't always blame the accts for tight brief...or loose brief.
Coz they too, have tough time with clients...to maintain agency/client relationship.
Sometimes if a creative idea tat client don't buy it despite advising/nego/persuading...no point venting
ur anger on ur accts...just let it go and work on the next brief.
Remember, if the client stay, you as a creative also stay unless u fug up the job.

[ Client ]
Since you are the only creative in the company, your presence/words do have some weight.
You hab to work closely with the marketing ppl within your organisation...and if your relationship
with them is good, everyday go lunch together...then things will go smoothly....even deadline can be negotiated.
The obstacle is shortened to 2 --> mkt head & boss. sometimes just 1.

Any doubts with the direction/strategies...you can clear it within a day with your colleagues without
going through multiple layers of decision makers. So things can move faster and you can plan your
work accordingly and most imptly, go home early & no last min pitches.

2. Creative output.
[ Agency ]
In the agency, you have different class of creatives...from designer to AD to CD. And it helps if you
need opinions to push the idea further but at the same time, this will also cause creative conflicts,
where great minds don't think alike.

Let's face it...sometimes CD's idea might not be the best but you still have to obey due to his/her status in the company.
However, if anything goes wrong, remember....prepare to take the blame. Don't expect your superiors will take for you.
When a good job is done, credit goes for the team.
When a bad job is found, blame goes to the creative.
Last but not least, scam work for awards.

[ Client ]
Yeah...this is your playground. Like Mars ranger said, you are the inhouse designer..
which means you should know your company branding/products/services well enough than any other people.
Whatever your ideas is, you can discuss with the mkting dept...and like i said, if relationship is good.
everything will go smoothly. Of coz, the product/services is always the same but creative approach
can be different. Do not restrain yourself, your own capability just bcoz is a inhouse.
Some inhouse do have wonderful jobs....eg. Zouk, ikea, etc.

3. Remuneration
[ Agency ]
Most of you can witness last year, lots of creatives tio sacked, contracts shortened, hiring freeze.
If you are not the top of the cream, prepare to play musical chair game. Agency is profitable
if they have big accounts and if they lose, why do they need so many head count?

You talk loyalty to them, they talk profitability with you. Forget about loyalty within agency.
The only reason why creatives like agency is becoz of faster, higher pay remuneration if you are welly good.
Jnr AD 2-2.5k and Snr AD 5-9k.. ACD, group head, CD is 5 digit above.
Bear in mind, we are referring to established agencies...and not 3-8ppl local agency here.
Small local agency pay like design houses.

[ Client ]
Ok...pay might not be as good as agencies but if the organisation don't go bust and you dun fug up
the work...chances of retrenchment is quite low. Coz you are the only few creatives...and they need you to churn work quick.
So that means is quite a stable job and your pay might increase when the time comes.
But then again, if your organisation do make good profits, you enjoy year end bonus.
some agencies don't even give bonus...

Of coz, all the above experiences are from my colleagues, peers and myself.
It might differ for individuals who dont face the same music as us.