Reviews of his arm strength run the gamut but from what I've seen, the consensus is that it's average to above average. One report called it "intriguing" because he "flashes good velocity" but then they still questioned his arm strength. From what I've pieced together, there's the opinion that he loses a lot when he's not throwing from a set base.
One thing about arm strength is that it is negated by having to step into and launch certain throws, which gives DBs more time to read where the ball is going and break on it sooner.
Haener does have to wind up like that from what I’ve seen in a very small sample, but getting quicker with his reads and having a specialist work on him with mechanics can certainly help.