The problems for me are in the PS4 OS itself, as well as missing options from all current gen games. Let me explain...
First problem for me is including multiplayer in the PS Plus subscription. It's not good value for me personally, as I only play a handful of racing titles. The net result is that I'm being charged to use my own internet, something which I'm not best pleased about, & don't want to pay for.
The second problem is that the PS4 doesn't support real time text chat. This was essential on PS3 for titles that didn't have in-game text chat, like GT5 Prologue for example. Having real time text chat on the console that could be accessed with one push of a button, without quitting or pausing the game, turned GT5P into a very sociable experience with those on my friends list as well as organized forum racing. Without that, it would've been a very lonely experience. Seeing as PS4 doesn't have real time text chat, I can't organize clean public racing like I did on PS3 with the three GT titles. Sure, you can have a conversation with up to 99 people, but you can't do it in real time, & have to quit the game to access it. Which is very clunky & all but unusable. This is essential for games that don't have in-game text chat like P CARS & AC.
Hosts not being able to kick dirty players. This is essential for clean racing in public lobbies. I did complain about it when P CARS first came out on the official forum, & Ian Bell said that he couldn't implement it because Sony wouldn't allow it. He had made a false assumption, because a little while later after much convincing from the community he was able to implement it. I don't know if hosts can do this in AC.
Not being able to name my rooms. In GT5/6 my rooms were named with enough information so people could clearly understand what was going on in them. Not being able to communicate what a room is all about is yet another thing that is killing online racing in P CARS in particular.
Not being able to change certain settings without having to leave the lobby & create a new one. In GT5/6 I could change any setting on the fly without having to start a whole 'nother room. The online in both AC & P CARS is Stone Age compared the the last gen GT titles.
On PS3 I was very quickly able to send multiple text messages to new players who entered the room. I could send saved messages in my outbox folder, which contained rules for the event & clean driving requirements, to multiple new players at once & within seconds. That was essential for clean racing in public lobbies, but I can't do this on PS4.
The net result of all this things is a killing of the social aspects of sim racing online in public lobbies. The various ways that the ability for hosts to communicate with other drivers has been butchered, or rendered so clunky that it's all but unusable. I cannot find anyway to continue the type of racing I enjoy, which depends entirely on having the tools to facilitate good communication with others.
The fact that GTS no longer allows hosts to set up tuning prohibited races is a killer as well, as that's all I used to do. It's a quick & easy way to race, but games designers are so dumb that they just don't get it. PD had some good options in GT5/6, options that they seem to care little about implementing in GTS, or even understand how much people relied on them.
Sorry for the epic reply, but I hope that answers your question!