I see where you are coming from, however one limiting factor on his roams is his Ult range which is at 2500. To put that in perspective, Swain's W (Vision of Empire) has 3000 range and honestly 2500 range does look and feel quite underwhelming, as normally even before your in Ult range they have spotted you out and are already retreating. Once Nocturne hits level 11 though, his Ult range increase by 750 (Now 3250 range) and it allows a lot more coverage over the map compared to the level 1 Ult range, this allows you to Ult prior to being seen by wards which allows for the easiest roams. especially since most roams pre-level 11 cannot go under enemy towers as they deal a surprising amount of damage, and after level 11ish normally the laning phase is over or about to be. Well I hope this answers your question some what, if anything is still unclear as to why nocturne cannot roam very reliably then PM me or reply to this! :)