When you search for monsters from VGtM, you see that all monsters are marked as "Legacy". The relevant dndbeyond page doesn't say much about it:

we may revisit older content in order to improve or otherwise adjust them. Going forward, any content that does not reflect the latest rules and lore will be referred to as Legacy Content.

Monsters from VGtM have their counterparts in Mordenkainen Presents: Monsters of the Multiverse. For example, here's Bodak, and this is "legacy" Bodak from VGtM.

The Wikipedia page says:

The digital edition on D&D Beyond is scheduled to be delisted on May 17, 2022, which corresponds with the digital release of Mordenkainen Presents: Monsters of the Multiverse (2022). Monsters of the Multiverse contains revised versions of the player races and monsters originally published in Volo's Guide to Monsters and Mordenkainen's Tome of Foes (2018).

But what was wrong with monsters from VGtM, exactly? Don't they "reflect the latest rules"? How significant the changes were? What problems can I face, if I use these "legacy" monsters in my own adventures? I haven't purchased Monsters of the Multiverse yet, so I can't check this myself.

When you search for monsters from VGtM, you see that all monsters are marked as "Legacy". The relevant dndbeyond page doesn't say much about it:

we may revisit older content in order to improve or otherwise adjust them. Going forward, any content that does not reflect the latest rules and lore will be referred to as Legacy Content.

Monsters from VGtM have their counterparts in Mordenkainen Presents: Monsters of the Multiverse. For example, here's Bodak, and this is "legacy" Bodak from VGtM.

The Wikipedia page says:

The digital edition on D&D Beyond is scheduled to be delisted on May 17, 2022, which corresponds with the digital release of Mordenkainen Presents: Monsters of the Multiverse (2022). Monsters of the Multiverse contains revised versions of the player races and monsters originally published in Volo's Guide to Monsters and Mordenkainen's Tome of Foes (2018).

Apparently, Volo's Guide is Legacy Content now.

But what was wrong with monsters from VGtM, exactly? Don't they "reflect the latest rules"? How significant the changes were? What problems do I face, if I use these "legacy" monsters in my own adventures? I haven't purchased Monsters of the Multiverse yet, so I can't check this myself.