To be honest I've never heard such big problems as those ... seems more like an exception than anything else? Sometimes when an update comes it might not work in the Steam version because the Social Club says "no, I don't want you to login" but other than that ...

But I can definitely understand your frustration, I had some trouble right after I bought it aswell.
But to come to the car customizing and modding again - now as I've played quite a lot, mainly online, I'm sad when it comes to the car customizing aspect. I'm not even in the mood of modding because firstly I know from GTA IV how hard it was compared to the III-era and secondly the cars in GTA V are (surprisingly) interesting enough that for the offline part of the game I don't want to switch them for real cars.
But the customizing itself is rather ... boring. Turbo tuning that costs a lot (not like you earn a lot in online) and brings like a 2% better acceleration, body kit parts are all 2003 Need for Speed Underground style and the wings mostly aswell. Wheels are
wayyy to expensive, as are colours. I don't even wanna spend the money on one of those new lowriders that just came out as you can easily pay two million to buy and modify one. But they're so slow... and not useable, just show cars which is fun with friends for like five minutes. I'd rather save the money and get a real supercar later on.
Rockstar said the tuning in GTA V would be near endless but honestly... the tuning in GTA SA was more in-depth as it is now, and that's hugely disappointing. It's not like expected that you can change so in-depth like in the new Need for Speed for example, but I expected more than just one bumper per car and an engine tuning that doesn't do anything...