To me, Lu Meng was more of a capable military commander but he wasn't that complete compared to Lu Xun (esp. if we're talking about diplomatic and political skills). Lu Meng gained Jing Zhou for Wu but at the same time, he put Wu in a very risky situation. On Wu's part, it was Lu Xun who stablized it. IMO, Lu Xun was more sensitive to Wu's needs than Lu Meng and the latter tend to do things based on short term gains.
To elaborate further, a lot of us tend to judge the abilities of officers using mainly military achievements. Hence, since Wu had only one province to start with, Lu Meng gaining another one (to be exact, part of Jing Zhou) would make him seemed to be the best person that Wu ever had. However, that may not be true if we look at the long term goals and some possible repercussions of his actions.
I would mainly focus on Lu Meng's expansionist policy towards Shu compared to a diplomatic approach. From there, I question Lu Meng's far-sightedness since I viewed his policy as a sub-optimal policy. Here're some points:
i) The alliance would be broken and Wu would need to face two enemies instead of one. I don't recall Lu Meng offering any advice on how to deal with that situation. He died when things were heated up critically.
ii) The only good reap off from betraying the alliance was Jiang Ling. I do not think that Ling Ling, Gui Yang and Wu Ling were worth all that. Still, look at the number of attacks Wu suffered at Jiang Ling. Wu had to spend considerable amount of resources to defend Jiang Ling and this burden could have actually been Shu's. (this is as opposed to the benefits which Lu Meng told Sun Quan, something like taking Guan Yu's land would allow Wu to control Yang Tze).
iii) After acquiring parts of Jing Zhou, Wu didn't have the capability to acquire more lands. IMO, it was not due to the inabilities of her generals but rather, Wu's resources had been stretched fully to defend her larger territory.
iv) As a result of Lu Meng's move, the battle of Yi Ling was fought. The results would be unimaginable had Cao Pi took on the offensive on either of them (and seemingly not enjoying his new-found title).
v) Wu had a long history of internal problems from the various tribes and clans. Could Wu cope with these problems while at the same time facing enemies from two or more fronts?