Of course Michael wants to play as many Firearm assets as possible to trigger his investigator reaction ability more often, but I wonder if this weakness is really such a drawback if you have means to - intended - draw (them) back. What it means besides crippling him if checked by the cops at an unfortunate time, it is for one highly probable that you have some more guns in your hand anyway (waiting to be played), but what's more, shuffling the ones in play into your deck means you get a free ammo replenishment on all of them!

I wonder if my 15 Permanent assets for my Mystic count...

I don't see any reason why it wouldn't. Like, I do control my permanents, right? Preston can manipulate resources on his Family Inheritance, so surely he controls his permanent asset.

Let's imagine the board state necessary to make it productive to play this card.

First, you need 5 damage and/or horror on you. This should be relatively common for investigators who don't have good soak.

Second, you need one or more in-play assets at your location that can receive up to 5 damage and/or horror. A Precious Memento could take all of it assuming it has no damage and horror on it. This should be relatively common for investigators with a lot of good soak.

Finally, in order to draw a card or two you need to be willing to discard one or more assets. This lets out expensive, self-healing assets like the Precious Memento and Jessica Hyde. You need to instead play with cheap, recyclable soak such as Leather Coat, Cherished Keepsake, Hunting Jacket, or a composure like Plucky. Drawing cards for these can help replace them quickly.

So to build around this card you should include lots of cheap soak. But instead of using it to soak your damage and horror, put the damage and horror on yourself. Then, instead of taking a draw action or two, play Wrong Place, Right Time.

Fun fact: This card can be used together of On Your Own, which can be quite nice as an additional soak with a neat effect. However, this card wants to be in a Blursed deck and the only investigators with access to both Tristan, On Your Own and Survivor Events are Bob Jenkins, Charlie Kane and Wendy Adams, and only the later really benefits from this Archetype.

Our table played this plant in one of the Hemlock Vale scenario and we had to double check whether we got something wrong, because everything is so uncharacteristically way too convenient, lol!

• Kills any enemy engaging with anyone at your location, or even not engaging at all like Aloof enemy. You just take a look at its HP box when someone drew it in Mythos Phase and throw it into the discard pile.
• Sure picking up Uncanny Growth is once per round despite the lack of exhausting, but its so even in very busy round you can stock it on hand and get a chance to do 2x Uncanny Growth next round. There is no penalty for not feeding it constantly or holding onto Uncanny Growth. The 1 cost of Uncanny Growth rarely matters when the Seeker also owns Dr. Milan Christopher. It also comes back to hand immediately if you fail instead of being set aside!
• Not even discard the enemy like Kymani Jones ability but it defeats, so you get Victory and other stuff like Microscope as usual!
• Exhaust is nowhere in the text box so it can even eat with stockpiled growth, do the Uncanny Growth thing, then eat again in the same round!
• Felt like it should have something that directs harm to its frail 1/1 soak, or threaten you somehow if you don't feed it. Turns out the soak is not even its weakness, it's a free goodies! Unless scenario card do something very funky, no way it'll get hit when Seeker likely has an Ally that takes an actual slot to take the hit. It doesn't have an Ally trait either so scenario card that are designed to bully allies missed this thing!
• It's not even checking printed health, it checks remaining health, so for some reason if your Seeker pulled harsh tokens and can't pre-collect enough growths earlier, someone can soften it up a bit (maybe with other automatic damages) and it's still testless munching time!

Maybe "limit 1 per deck" is only its real limitation, or how the Seeker can sometimes (unlikely) lose a held-on Uncanny Growth to a treachery that discard it to discard pile and can no longer "search your bonded card" to add it back to hand. The hoops it needs to get its unlimited testless enemy management going is very easy, compared to something like Nephthys.