The Numbers:
1Star Zaku hovers around $525, although it has touched as high as $700+ on occasions. This is in the same range as the other 'top tier' summoners, such as Scarred Llama-mage and Crypt Mancer. The low-volatility of both his price during a time when other cards were dropping 50% makes him a good candidate for a straight purchase. But what sets YoZa apart from the rest, is his insanely high rent ROI. As of this writing, a 2star will give you an almost 55% ROI and a 1star will earn around a 74% ROI, assuming you can rent him out all year at current prices.
Risks:
The biggest risk is time and its impact on the value of the card. A year is an ETERNITY in the crypto world. As new sets come out Zaku could see his viability in the meta fade or just get outshone by newer summoners, impacting his price. Also, Splinterlands has said they plan to disallow older sets from ranked, possibly only allowing them in a 'historic' type format. Obviously this would tank the price, however the Splinterlands team has show on multiple occasions they don't want to alienate their older players and Untamed is not too old of a set. With that in mind though, it may be prudent to not put all your eggs in one basket. Remember, this is a game, not an investment. The rules can change at any moment.
A second major risk is that Zaku's rent price could decrease dramatically, either as a result of less new players joining the game, a change in the meta or even a change to how rewards are distributed. For example, Mylor Crowling rental prices went from ~40DEC/day to ~20DEC/day over the past month. His price and age made him less of a secret weapon, and players learned to play around thorns. Influencers stopped talking about him. To top if off, rewards were lowered in bronze league, making botting less profitable. Some bots used Mylor, but mostly bots were easy to farm if you were a human player doing nothing but renting Mylor. Something similar could happen to Zaku.
Even taking into account Zaku's risks, with his incredible rent return and playability on the days you aren't renting, I give Yodin Zaku a rating as a 5/5 Strong Buy.
*I am not an investment professional and this in not investment advice.