Currently under the conditions of this shiny beta test HC of sorts, it’s not only possible but more likely than not for a player to encounter a shiny only to immediately fail to recruit it.
In other pokemon games where shines exist, it’s possible to fail to capture a shiny as well. However, this is usually due to player error rather than failing a coin flip; to my understanding, shiny spawnrates are already planned to be higher than the main games to compensate for this, but I believe this system is likely to create a lot of frustration for people deliberately hunting shinies as well as players–especially newer ones–who bump into a full-odds shiny on accident and more often than not completely whiff the encounter.
Additionally, a higher spawn rate also means anyone who keeps a pokemon gift on their person at all times is now operating in a system where the chance of an individual pokemon being shiny is higher, the amount of pokemon that can be encountered per hour is significantly higher, and they can guarantee the recruit that was supposed to be a coin toss to balance the above two factors out.
In order to both reduce the frustration for players caught in the former scenario, and avoid the potentially ridiculous min-maxed odds in the latter, I propose shines outside of this event be made to have a 100% recruit rate when spawned but have a reduced spawn chance–even lower than they do in the main series. Say, instead of encountering 8 shinies and recruiting 1 of them over a long period of time, the player would encounter only 1 within that same timespan but be guaranteed to recruit it. This would preserve their rarity–which is important–as well as reduce the chance of players winding up in the soul-crushing situation of the game giving a player false hope and then crushing it when they fail the coin flip, instead turning a shiny encounter into an extremely rare but equally rewarding high moment for the player. This also prevents the pokegift and recruitment boosting items from significantly screwing with the balance of shiny hunting.
EDIT: If the above is confusing and sounds like I want shinies to be easier to obtain, read this post: Shiny Spawn and Recruitment Rates
EDIT2: Concerns have been raised regarding a flat spawn rate for shinies regardless of progression. As such, in addition to the above, I propose the spawn rate of shiny pokemon be weighted by the explorer rank of the party leader–higher ranks increasing the spawn rate. (With a further decrease to the base rate if deemed necessary)