I don't think he's a major injury risk really (based on how he moves and his build), but when you're debating how high his ceiling is, that's the sort of thing that comes up. Part of what made Lebron the best prospect ever is that he had a legit NBA body when he was 16 years old and seemed like a very small injury risk. Obviously anyone can get hurt, but he came into the league measuring 6'8, 245, moving extremely well for his size. Wemby is going to put on some weight, and just can't claim to be as much of a sure thing to stay healthy as Lebron was.
The sidekick thing is a real concern. Lebron came into the league with a bang as well - he was 2nd team all-NBA by year two, second in MVP voting by year three, undisputed best player in the league by year 6. Still, in his first seven years in Cleveland, they didn't get seriously close to a title. One finals appearance ending in a sweep, and one other EC finals that they lost 4-2 (but might have been a sweep if Lebron hadn't hit a last second miracle shot). It's not hard to see why. Lebron didn't play next to a single all-NBA player in Cleveland, and only played with 2 pretty pedestrian all-stars (Ilgauskas in 2005, Mo Williams in 2009).
It does seem like a lock right now that Wemby will be a top 5 player (at least) for the next 10-15 years, but that absolutely doesn't mean that the Spurs are about to get a bunch of titles. In a hypothetical world in which we're packaging multiple players for Wemby, that's something to keep in mind (Jokic/Murray could easily end up better than Wemby plus whoever his best teammate his, as could SGA/Chet/Williams).