It looks like the UA Dragonborn is an upgrade of the Players Handbook Dragonborn.
The Breathweapon damage bumps up from 2d6 damage to 2d8, and its frequency changes from per-rest to proficiency-per-long-rest. The change is probably a wash at low levels, but more frequent at high levels with ability to "nova" (to use up all uses in one encounter). Plus there is an additional new trait at level 3.
Generally, the upgrade seems to fill out the Tashas Custom Lineage design space in a beefier way. Its equivalent slot-1 spell is more satisfying − and uses the missing Darkvision space for an additional trait at level 3. But the additional trait varies in value, according to the kind of Dragon.
The Chromatic Dragonborn (black, blue, green, red, white) Chromatic Warding trait is meh: Damage Resistance becomes immunity temporarily per-long-rest.
The Metallic Dragonborn (brass, bronze, copper, gold, silver) Metallic Breathweapon trait is good: a choice between push or incapacitate per-long-rest.
Going beyond the Players Handbook, now the Gem Dragonborn (amethyst, crystal, emerald, sapphire, topaz) is available. It has two extra traits. One-way telepathy which is meh ... even annoying. But the main Gem Flight is amazing. The flight is for 1 minute per long rest, at walking speed. Altho Gem Flight is slower than the slot-3 Fly spell (generally speed 30 versus speed 60), it requires no Concentration, so can be even better than the slot-3 spell.
I suspect the Gem Flight trait evidences the designers rethinking the role of flight, and what tier it fits in. It looks like low tier access to flight has become more acceptable. This means, low level adventures will take flight into account with regard to hostiles with missile weapons, the kinds of traps, and so on. Probably, flight is more acceptable because of the 5e Concentration mechanic, which prevents layering too many powerful spells (including flight) simultaneously. Apparently flight plus one powerful spell is balanced enough.
Relatively speaking, the most powerful Dragonborn is probably the Amethyst Dragonborn, whose force damage Breathweapon finds less resistance, and who has flight. Depending on how much use one gets out of flight, this lineage is either solid or the best compared to the Players Handbook lineages.
The Copper Dragonborn is also competitive, whose acid damage Breathweapon is decent and whose Metallic Breathweapon incapacitation can be effective to help take down a boss, or whose push can be situationally useful.
I consider the Metallic Dragonborn an excellent example of the Tashas Custom Lineage.
The Gem Dragonborn is beyond it and the flight is a wild card.