We have a public playtest...of exclusively player-facing content...so the only metric of monsters/challenges for us to compare this player content to...is the old stuff we have.
And then... we're informed they are changing some things in their monster/challenge design... and they are exclusively internally testing their new monsters and encounter building... why on earth? EDIT: "exclusively" is my own cynical take, not verbatim.
Regarding monsters for playtesting against.
The monsters in
Mordenkainen Presents Monsters of the Multiverse are the most recent update of the math of 5e.
This product is compatible with 2024 moving forward.
Even if the designers tinker with some monster features, or add or delete monsters, the fundamental math of 2024 appears to be Mordenkainen. So it is a suitable source to build playtest scenarios, until we get more info about 2024 monsters in a future UA.
For example...they talk about honoring the history of D&D on its 50th anniversary...and we still have low level spells like Goodberry or the heavily powered-up 5e Leomund's Tiny Hut that are radically disruptive to exploration styles of play which were historically a big part of D&D. Did we ever get playtest material addressing these spells? No. Did we ever get playtest material reimagining the scant exploration rules for a modern audience? No.
This valid issue, seems more to do with designing a setting. To remove official options and to add variant or indy options is normally a necessary part of DM worldbuilding.
So, discussing which spells are or arent appropriate belongs in the DMs Guide. (Along with which species are or arent appropriate, classes, backgrounds, equipment, skills, toolsets, etcetera etcetera.)
Regarding spell balance. I am delighted the designers have been updating various subpar (and a few problematic) spells with more powerful (or more coherent) versions. I am even more delighted that feedback is majorly positive.
2014 did a good job of removing the overpowered spells from earlier editions, but left many subpar spells in place. At least some of these subpar spells are getting a boost. I hope all of them do.