5th dnd – How strong is the tressym compared to other familiar options?

I was looking at the description of the tressym in the Thunder King's Storm adventure, and saw this (p.243):

With the DM's permission, a person who launches the find familiar The spell may choose to bring up a tressym instead of a normal cat.

The tressym seems really strong in comparison to other pets, such as the owl (which many consider to be the strongest pet).

Tressym compared to the owl:
begin {array} {r | ccc}
text {} & text {Tressym} & text {Owl} \
hline
text {Stats} & text {-4, +2, 0, 0, +1, +1} and text {-4, +1, -1, -4, +1, -2} \
text {Skills} & text {perception +5, stealth +4} & text {perception +3, stealth +3} \
text {HP} & text {HP (5)} & text {HP (1)} \
text {AC} & text {AC (12)} & text {AC (11)} \
end {array}

The owl has a flying speed, but it has almost no climbing speed.
The tressyman has the feature Detect Invisibility (with a range of 60 feet) as well as immunity and poison detection. These features, compared to the Flyby ability of an owl, seem not worth it.

Is it as strong as I think or do I neglect something?