Nils, I am open to discuss the point of versatility vs specialisation for layman guilds and how it could evolve.
Changes were made to layman guilds when I did the relatively recent global review. A purely offensive or defensive layman melee guild could make use of their full combat aid cap for their special... so minotaurs, blademasters, pirates are examples of that. Melee guilds that can switch between offensive and defensive abilities (like lay AA, Templars or Thornlin militia) have a lower maximum combat aid cap allowed for their individual abilities compared to pure layman guilds. So pound for pound they won't be offensively or defensively better than a 'specialist', but they do get the benefit of a bit of both. On balance, they all have the same combat aid.... and I think the layman combat guilds are generally in a good balanced place.
Layman magic guilds do take versatility to another level, and they probably will have to evolve further once other global considerations come in to play in the game. As part of the layman review, the maximum combat aid cap of maintained spells (individually and stacked) was introduced. That cap for maintained spells makes them more aligned to melee generalists now. Granted, still with a lot more versatility... but also with a lot more requirements to use those abilities than melee guilds. Does that make them balanced though? Does that argument hold water?
From my perspective, I look at how the layman magic situation has evolved over time. After the introduction of the standardised magic system there was only one layman guild created using that system... and it was the only one for a long time that used that system. That was the Worshippers, and they became the de facto benchmark for future layman guilds. As we began the conversion over of the other layman guilds (Necromancers, Heralds, Minstrels) the balance issue was more about who could access what layman magic abilities.
So the Warlocks were born as a baseline balance to the Worshippers, allowing access to essentially the same layman spell abilities to just about all guilds and players. These two guilds have a spell toolkit unrivaled by other layman guilds. And correct me if I have misinterpreted.. but I imagine your issue with 'master of all trades' isn't about all layman magic guilds but about these two guilds in particular?
From a balance point of view, they will need to go through an evolution to deal with the issues you've raised. Those two guilds will need to become more specialised and less jack of all trades. However in my mind the game is not ready for that yet. I feel there needs to be at least one solid evil layman healing option to allow enough playing options. Once we have that I think there can be a tightening of the abilities of those two guilds without leaving a capability gap for evil players.
Just a couple of other points.
All layman guilds fit within essentially the same combat aid limitations. And combat aid is based on combat benefit at any one point in time. At any one point in time they are all equal. So they are all balanced (for a given value of 'balanced' [paraphrasing Pratchett's "for a given value of 'truth'" quotehttps://en.wikipedia.org/wiki/Lie-to-children
When talking about balance, guild tax is a distraction. It is a secondary or tertiary lever these days when considering balance. Combat aid is king.