Sideboards contain the wrong cards in Bo3 matches:
https://feedback.wizards.com/forums/918667-mtg-arena-bugs-product-suggestions/suggestions/48694358-bug-on-sideboard-inside-best-of-3-games
(Traditional Draft and Sealed are temporarily disabled because of this.)

You can’t always see the contents of graveyards when you need to:
https://feedback.wizards.com/forums/918667-mtg-arena-bugs-product-suggestions/suggestions/48693707-graveyard-priority-lag

Tender Wildguide doesn’t hold priority (but its Offspring token works fine):
https://feedback.wizards.com/forums/918667-mtg-arena-bugs-product-suggestions/suggestions/48693464-tender-wildguide-not-stopping-between-phases