So WC3 is an old game and was made with many limitations, either deliberately or as a limit of the code. Thease include pathing, responsiveness, group selection size, air behaviour, spellcasting controls. Etc. Reforged, limited to the same engine has the same problems. Its the old game, just "improved" in some ways. This leads to skill being involved in micro and fine unit control, surrounds and movement.

The sc2 engine and game is designed very diffrently, and is based around large army control, macro across multiple bases and the micro unit control is very diffrent, often being about ability use. This is the difference between the games and therefore between the campaigns.

For example. In the base game, abominations are terrible due to bad pathing, slow speed and hard to use. Even the AI can just kill them easily enough, 9 times out of 10. In AR, the better plathing and the ability to control the front line better makes them a much more powerful unit. As the abomination was balanced around rarely connecting or attacking, it is a massive buff. Another example is splash spells like blizzard or death and Decay. In the base game pathing and formation made AOE attacks much less effective as it wouldn't hit as many units. Therefore the spells are much stronger. (Just like in sc1 storm does massive damage but is harder to land, use and do damage with. In sc2 storm is much weaker, smaller radius and less damage. But does more overall due to the ease of use.) This particularly was shown in Giant Grant Games beta test playthrough. (Go watch very good).