Mighty summoner does not apply to non-spell summonings

The relevant line from the class feature (XGtE, p. 24; emphasis added):

Any beast or fey summoned or created by a spell that you cast gains the following benefits:

The feature is missing any clause, such as "... or other feature/magic" which would have the effects apply to creatures summoned by features.

Mighty summoner does not apply to non-spell summonings

The relevant line from the class feature (XGtE, p. 24; emphasis added):

Any beast or fey summoned or created by a spell that you cast gains the following benefits:

The feature is missing any clause, such as "... or other feature/magic" which would have the effects apply to creatures summoned by features.

As a side note on the listed examples; Hound of Ill Omen would not work because it is also a monstrosity, the Accursed Specter is undead, and the Drakewarden is a dragon.