# [RPG] How strong is the tressym in comparison to other familiar options?

balancednd-5efamiliarsmonsters

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

With the DM's permission, a person who casts the find familiar spell can choose to conjure a tressym instead of a normal cat.

The tressym seems really strong in comparison to other familiars, such as the owl (who many see as the strongest familiar).

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

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

Is it actually as strong as I think, or am I overlooking something?