The learning curve in general is just steep. It takes a lot of patience and restraint if you want to win because unlike most shooters it's about survival, not killing so you've got to train yourself not to take unnecessary risks, not take shots that you don't need to so you don't give a good position away, ect, ect. On top of that it takes a lot of time invested to lean the map, learn the guns, how they handle with what attachments, ect. Especially in regards to learning the weapons because you don't get a ton of opportunity to use them. Learning how to read terrain and find cover where cover isn't obvious takes a lot of time as well.
Even playing with friends you'll lean pretty quick that some play styles in this game simply aren't compatible.
Solos are my least favorite. I just find them boring playing alone. Prefer duos because you get to play with someone, aren't competing for gear, and it's a lot easier to face a duo as an individual than it is to go up against a whole squad. I play Squad mostly, but only because my gaming group has so many players.
I think the worst aspect of it for me is hit detection. Blasting someone point blank with a shotgun only to not kill them because it detects you hitting an arm before the torso sucks. I have the WORST luck with snipers too. I get one shotted in the head every damn time but I can nail someone in the face with the K98 twice and never down them it seems.