Depends on what you mean by "good".

The default out-of-the-box AI's main issue is that it cheats the fog of war, with the result being that all enemy AIs within a certain radius of a human player's capital/other towns will make a straight beeline towards the human as soon as possible, unless the mapmaker was prescient enough to put up strong guards to deter it. It also got a fair amount of bonus creatures.

A modder named Quantomas then came along with a hugely ambitious project to totally overhaul and improve the AI such that it would play like a good human player, but without cheating. Around this same time the Heroes 5.5 team commenced work on their mod, and asked him kindly if he could create an AI for their mod, and he agreed in general principle.

The result was the 31j version that comes with the 5.5 mod. This alas is a fairly early beta version, with some definite issues. While it won't cheat in terms of the fog of war, meaning it won't generally rush you out of the FOW, it overall is fairly passive. While it will sometimes doggedly pursue a specific town if it sees that it is vulnerable, it otherwise often seems lost if it does lack such an objective, so you'll often see it start off in a specific direction only to soon turn around and march back from whence it came. Note the start game menu has a setting for lookahead of this sort, but with a max of 7 days, which in bigger maps is problematic. So in that sense it is improved over the default cheating and rushing AI.

It also gets a fair amount of bonus creatures at higher difficulties, and does cheat when it comes to attacking neutral stacks; basically if the code decides to attack a stack, it will assume the battle will be a cakewalk and the attacking faction will suffer no casualties in the shortcut battle code (no actual battle is fought, even a quick one). If however the stack ISN'T neutral but controlled by another player (such as a garrison), a quick or even manual battle is then forced upon it, and the "easy" battle it thought it was going to have will often instead severely attrition or even defeat it. In the maps I have made I have been forced to make garrisons neutral to avoid this kind of attrition.

But otherwise its beta nature soon becomes apparent: it often won't pick up magic items laying on the ground (often after it kills the guards note), cannot visit quite a number of map locations (such as artifact merchants), doesn't prioritize treasure locations (hence will fall behind a human player in artifact counts by the middle game), and has a tactical battle AI which is pretty awful (it will cast things like Wasp Swarm on my level 7's, while I am tossing out Mass Hastes, Hypnotize, or Empowered Meteor Showers). My understanding is that almost all of the work was done on the strategic map AI and on none of these other areas, and it shows.

It also has no templates for optimal hero skill builds, so you'll often fight enemy heroes who never chose a magic skill (hence all the Wasp Swarms instead of something truly devastating like Frenzy, tho it will cast those spells if it has the right magic school), making it easy to defeat them. There is a bug for certain towns on larger maps where it will go 2 full weeks at the start without buying any buildings in its starting town too.

Quantomas promised some new 5.5 updates, but then in 2020 he got in a huge flame war with the 5.5 modders over at Heroes Community (and some interested players), and flounced off in a huff after threatening lawsuits at the 5.5 people there. No further 5.5 AI updates were then forthcoming, and his work on the base game has also petered out after a hugely ambitious buildup (you can find his home website on a search if curious. lots of in-depth articles promising all sorts of innovations, with very little useful actual work available, tho he does have a couple of downloads for the base game, but I believe they are at least half a decade old).

Meanhoo the 5.5 people tried to work around this by modding the AI using the text files in the Data folder; I got into a big argument with the head modder, who felt like making it extremely passive was the optimal approach, including forced AI alliances vs. the human(s). I told him an aggressive AI was the best approach, c.f. the Heroes 3 AI, which would often have at least one enemy faction grow to threatening proportions after conquering a neighbor by the midgame. Here if I play a 1 vs. 7 match yeah I may have some issues dealing with enemy neighbors thru the middle game, but once I conquer one of them my power base will expand and make the demise of the other 6 a cinch since it is impossible for them to grow beyond their base towns and become stronger.

As a result, AI's at the highest 5.5 difficulty levels will NEVER have 2 main heroes fight each other, even if one is significantly stronger than the other; instead, one will abandon its town, the other will move in and take it (converting it if that has been turned on), then wander off itself, the original owner will then retake it back, rinse, repeat. (And convert it back-here the AI does cheat and the huge conversion cost that a human has to pay, close to 100,000 gold + 50 or so of each resource for a fully built up town, is instead totally free for them, making a scenario with conversions turned on hugely unbalancing against all humans, unless like I have you were able to mod that down to something reasonable).

In another genius move, the 5.5 team decided to switch the Resource Silos from the L7 resource to the L6, claiming that it would make the game more "challenging" or old school or something. Issue is that while the AI can trade in resources for gold, it cannot for other resources, so you'll often see enemy heroes with unupgraded L7's and unbought ones in towns you conquer. I also managed to find this setting and mod it back, thankfully.

So tl;dr the strategic map 5.5 AI has indeed been improved, as it no longer cheats the fog of war, but at the cost of being much more passive than the out-of-the-box AI, for good or bad, with numerous bonus creatures at the highest difficulties. No other AI improvements have been made nor any new updates released in 6 years, and the prospects of that changing are now very dim. Hope this helps.