There is a lot of hype about Watson. I don't agree he is one of 'those guys'. Also there is a degree of variance that comes down to chance. While I use this 4 to potentially 5 game mark, it also has diminishing returns on both ends. If you have a team that was 12-4 with a bad QB, getting them a really good or even great QB shouldn't make them 16-0. Just like if your team is really bad with a great QB and went 4-12, getting a JAG shouldn't automatically make you 0-16. It is hard to do either of those things. The easiest gains and losses to be made record rise have always been between 4-12 to 12-4 Once you start to go above or below that, it gets harder to go in either direction.
The fact Watson went 4-12 shows just how bad the Texans were, and also shows his play was a bit overrated last year, as he is. It also shows he just got unlucky.
Generally here is how I see QB value
Upper end HOF QBs 4-5 games Brady/Mahomes (arguably top 10 all time kind of guys)
lower end HOF QBs/borderline 3-4 games Kelly/Moon or Phillip Rivers
Really good QBs 2-3 games (Watson is the upper end of here)
good QBs 2 games (Stafford/Ryan,Mayfield)
serviceable starting QB 1 game (Goff/Trubisky)
bottom tier starter/high tier back up 0 games (Dilfer)