IMO, the only really top crossers in the world currently are Maicon and Dani Alves, both are specialists, and have been proven over time. Abate isn’t even close to be a great crosser, but he IS a decent crosser, much more decent than what people like to make him out to be. I see him hitting at least 2 crosses on target in most of Milan games (and I make sure that it isn’t just an eye test, I tell it to the person who I’m watching games with to keep count

), he can hit them, and I believe that if Milan had more movement inside the area (taking advantage of someone's physical gifts), and a different style in midfield his ratio would go up, maybe not by much, he would remain in the “average”, but it would be more obvious (cause It would end in goals) by the naked eye of fans that watch the same team week in and week out (instead of occasionally watching other teams) and who prefer the grass of the other side simply cause Abate didn’t cost us 20 mil + no team fought for him in the market.
Fact is, most crosses go either to the other side of the pitch, end up in the keeper’s hands (a minority ends up in the stands, but there’s usually 1 per match that ends up there), the majority ends up in the defenders head (c’est la vie, the defenders have the advantage there) very few end up in the desired target, and most of the ones that end up in the desired target are due to…you guessed it, good movement from the forwards.
Accurate crosses per team (first two fullbacks per team, players with over 15 games)
Real Madrid
Marcelo 0.4
Arbeloa, Ramos, Coentrao 0.2
Barcelona
Dani Alves 1.1
Abidal 0.2
Puyol 0.1
Valencia
Mathieu 0.9
Jordi Alba 0.5
Espanyol
Dídac 0.7
Juventus
De Ceglie 1
Lichtsteiner 0.8
Inter
Maicon 1.7
Chivu 0.3
Napoli
Dossena 0.9
Maggio 0.8
Ac Milan
Abate 0.6
Antonini 0.4
Arsenal
Sagna 1
Andre Santos, Gibbs 0.3
Chelsea
Bosingwa 0.8
Cole 0.5
Ivanovic 0.4
Man Utd
Evra 0.4
Phil Jones 0.4/Smalling 0.3
Man City
Clichy 0.7
Zabaleta 0.4
Tottenham
Bale 1.3
Assou-Ekotto 0.7
Bayern Munich
Lahm 0.4
Rafinha 0.4
Borussia Dortmund
Schmelzer 1.1
Lowe 1.1
PSG
Jallet 0.6
Tiene 0.6
Montpellier
Bedimo 0.7
Bocaly 0.5
Lille
Debuchy 0.8
Bonnart 0.5
Ajax
Van der Wiel 0.3
Now, truth is it’s unfair to say this fullbacks suck or are terrible at crosses because of this, it's just a ratio, I'm not using the quantification of amount of crosses they make so there can be a sample problem in this ratio (though I tried to use fullbacks or players disguised as fullbacks who played over 15 games cause most of them will have a certain big amount of crosses done overall).
And obviously, there’s other things that come into play. For ex. Madrid plays more to Di María + Cristiano crosses than their fullbacks, more or less same goes for Bayern, Barcelona overuses their left side cause their RBs are older and can’t handle it (though also cause much like with Inter, Maicon-who apart from this has turned into a ghost in everything else as a fullback in the last 2 seasons- and Dani Alves are the only crossing specialists in the world and it’s obvious to take advantage of it, if the other teams had specialists like this two, they would play to them).
Juventus plays A LOT through crosses, mostly cause of Pirlo + the fact that they don’t have any other type of forward other than CFs. We all know Napoli uses wingbacks, so its also logical they have more accurate crosses. There’s some of this fullbacks that operate both as wingers and wingbacks (Bale stands out) and not so much as fullbacks (just disguised as such)…so also their ratio is bigger.
There's also a thing that I doubt I'll find anywhere, crosses aren't just the ones that go up to someone's head, they can be lower crosses to the feet inside the area, or they can be directed to outside of the box (with players that can take it from first intention). From watching Abate I would say high crosses (aimed to the head, you know the ones we usually see more cause they're the ones mostly used) are his weakest link (sadly yes those are the ones he tries out the most), BUT his lower crosses are actually well intended (which shouldn't be surprising, he is a good passer) most of the time (this are the ones he pulls more consistenly but doesn't do as often). But oh well that would only be interesting to look at one day.
Thing is, back to the original point, taking away different playing styles not just from the teams, but also the leagues, you can see that there’s an average range, between 0.4-0.8 where most fullbacks locate themselves in terms of accurate crosses. Like I said, if our movement was better inside the box, Abate’s accurate crosses would increase IMO to a 0.8 ratio (doubt it will get to more), still remaining in the average, but it would be easier to look cause likely we would be scoring off from them much more often than once a season. The crosses are being done, their accurate on average ranges for a fullback ("decent" if you will, nothing magnificent nor close to it but certainly not crappy), the thing that’s missing is that we're not scoring off from them.