I find FW background kind of dry; there doesn't seem to be as much life to the forces with their writing. The battle stories I can leave from both, as they're generally less interesting.
The rules are definitely not as polished as GW main ones, which is saying a lot.
Both are generally less interesting than the early second edition, late Rogue-trader stuff that solidified the setting. Early rogue trader is fun, but less recognisable now.
And also this.
FW is marines. And guard. And marine fighting marines. And marines and gaurd fighting guard. And guard and marines, fighting marines and guard.
With a tiny smattering of xenos in there. Occasionally. FW books;
1 - Guard
2 - Guard and Marines
3 - Marines and Guard vs Tau
4 - Guard vs Nids
5 - Guard and Marines vs Guard and Marines
6 - Guard and Marines vs Guard and Marines (with a little chaos)
7- Guard and Marines vs Guard and Marines (with a bit more chaos)
8 - Marines and Guard vs Orks.
9 - Marines vs Marines
10 - Marines vs Marine
11 - Marines and Guard vs Eldar
Heresy - Marines vs Marines
So yeah FW are "grown up" if you only want power armour.
Autarch, Shas'o, Chaos Lord and Decadant Lord of the Webway. And a Doctor!
http://drlove42.blogspot.com/
Bonus if the gaming store in your mind has Saloon-style doors that B.A. can throw open when he enters.
Also, I think the OP confused the word observation with the word opinion. FW and GW have a different focus, for sure, but it is still a game set in the future with aliens and Space Marines and such. I, for one, don't want all that much realism in it for the same reason I don't watch law, police, and medical dramas on TV every night: when I get entertained, I want to check reality at the door and escape for a little while.
Rather than declare what you like mature, and what you don't like for the kids at your local store that annoy you, realize that there are people of all ages that like all kinds of different things. You seem to like military history, and that is the crowd that FW caters to, not adults or mature players. In fact, I have played against 15 year old players at various stores with wonderfully done armies that were good sports, and I've seen middle-aged men throw a fit and pull their models off the table in the middle of a game and leave. Age has nothing to do with anything.
Having said this, I like the models that FW produces, and have a few myself. I hate their production quality, which actually seems to have gone down considerably over the years.
I suppose FW does cater to a certain type of gamer.
By their prices I assume its the type of gamer that enjoys a good bend over session while he bays like a pony.
I love the Forge World stuff, I just wish I could afford it