Buff-focused playstyles are difficult to make "fun" for everyone involved because they have several potential issues that other playstyles don't really struggle with.
Aside from the obvious issues like buffs being too strong/weak, too easy/hard to apply, and base units being too weak/strong, which will obviously frustrate one of the two players, and potential design flaws that result in annoying units because they are only usefull within the larger framework of buffs (e.g. Saurus guard and Skink footheroes are purely vehicles of buffs, but mostly useless on their own, or skinks only being usefull offensivly with buffs stacked to the heavens but useless independently). There is one very weird issue; namely the fact that if you somehow did get the balance perfect buff-armies are inherently kind of frustrating to deal with in a PvP game.
The basic issue is that, in theory, as long as your buffs hold, you should be at a significant advantage.
While if you lose your buffs you should immediately be at a major disadvantage.
Essentially, you are never actually on equal footing with a non-buff playstyle.
This means that any game with an army with this playstyle immeadiatly forces both players to constantly play (nearly) perfectly, because a lucky roll at the wrong moment, or a slight mistake, can have great consequences, which quickly devolves into a loss (or win depending on your point of view).
This is especially problematic at lower levels of play, where players need, and expect, some freedom to make mistakes because they simply aren't perfect. If they don't get that freedom, players end up frustrated, and often feeling like the match was unfair, or that a particular unit/ability/faction/whatever is OP.
This inherently makes buff-heavy playstyles a bit of a problem.
Also, if you're wondering why this is an issue specificly for PvP games; in PvE you don't have to worry about the enjoyment of the AI, so problems like the buffs being "too easy" to apply for the player don't necesarly matter as much

.