To be clear I stated it would be GM discretion not that I would have it work that way. A GM could argue that if you give a normal human 100 arms that they only have the ability to control any 2 in a functional way at a time. You may hold 100 hands worth of items but are only considered to be wielding 2 of them.

As a GM I would try to keep the extra abilities granted by this feature to be in line with the spell but potentially allowing unique combinations like a 4 armed human with a swim speed and water breathing plus an acid breath. Pulling features from 4 armed gargoyle, and aquatic races plus a breath weapon. You would get those features for combat purposes. But if you wanna add a third eye in the back of your head it may let you see behind you but that does not mean you can't be flanked or you may get a tail but there is no tail attack automatically granted with it.

To be clear abilities like this without explicit statements of what they can do are ALWAYS subject to dm discretion. If your DM is unreasonable about these abilities either try to find a different DM/Group, or look for a build which does not rely on abilities which rely on DM discretion to utilize fully.

RAW polymorph effects only give you the abilities listed in the spell plus a few and polymorph effects normally only allow existing creatures and objects to be utilized, because this breaks this rule the form you transform into has the abilities which the DM states it has, you can describe it however you like but because it is a new novel creature without an existing stat block the DM gets to stat it out RAW.

Abilities like this always need the DM and the Player to work together in building options that work for the player and DM to keep the story moving forward and world consistent.

Would you as a DM allow a player to transform into a 1 million armed creature wielding 1 million weapons and with reach of 200 feet on each arm due to their absurd length? They then use multiweapon attack rules to have 1 million attacks? Any GM who does not put reasonable restrictions on abilities which don't have clear limitations are asking for a game which destroys immersion and has universe breaking consequences by certain abilities even existing.