One of the big impacts of the UA was to make parties of a certain level more powerful, mainly through Cavaliers and Fighter weapon spec.
I'm running a 1E campaign at present and I allowed some bits of UA, single class fighters have weapon spec, and we have a Cavalier. I also used some of the new weapons, the new rules on Demi-human level restrictions, the new spells, and a few of the new magic items - I don't see any real problems with these. I did however veto the UA stat generation method and the extra races.
The net effect however of Cavalier + Dwarf Fighter with double spec in warhammer has been that they have just smashed their way through both G1 and G2 without *too* much difficulty - at a slightly lower level than the modules state, ok they had some issues dealing with environmental aspects, but the casualty rate was lower than expected. G3 is another beast entirely (hehehehehehehe) - however they are making slow progress and are now near the end of it.. (after xp from G1 and G2 the main party is Cav 10, Dwarf Fighter 9, F/MU 5/9, Cleric 9, MU 11, MU 9, Druid 11, Thief 11 - with occasional appearances from a Ranger 7, and a Human Fighter 7 (with 2H Sword weapon spec).
But UA has something great, something iconic - and that's the appendix on Polearms..!!! I also think the unarmed combat rules are better than those in the 1E DMG, but that's not difficult.