In my opinion, mana is neither an inherently good neither a necessarily bad mechanic. Taking a guess I would say it is attributed to the fact that EU originally is an adaption of a board game in which mana simply represents action points per turn. For a board game that is fine, for a RTS game it depends on implementation.
The main benefit of monarch power is having a fairly natural and healthy limiting factor that prevents the player from steamrolling too quickly and at the same time is not entirely frustrating or makes the game too linear. In the Total War series for example the games are very much about the economy and if you can financially support a doom stack you can go from one to ten provinces in nine turns. Good implementations of monarch power usage in EU4 are in my opinion coring and culture converting as it takes time to reap the benefits of your investments. Bad implementations are cases in which you can store monarch power and seemingly retroactively obtain an immediate benefit. Examples here are buying up stability, buying down war exhaustion and buying technology. These are bad implementations precisely because they allow you to make problems disappear if you just wait long enough rather than actually having to plan ahead and having to deal with a problem. What is the point of having a decision that costs you five stability if you can just wait long enough and make the problem go away instantaneously? Imperator in my opinion is a game that has a lot of these bad implementations. As an exaggeration you could say in Imperator as long as you have monarch points you can do anything, if you don't, it turns into a waiting game. In EU4 some issues could be easily removed by introducing a cooldown: What if you could only buy up stab, buy down war exhaustion only once per year, buy technology only once per month in each category? Even better in my opinion would be mechanics that would force you to spend your power gradually. For example you could remove the option to reduce war exhaustion and instead put in a box you can tick to grant you a sizeable reduction to your monthly war exhaustion gain for say 4 dip points a month. Similarly you could have options "stability effort" and "work on technology". Being able to retroactively apply monarch power for an instantaneous benefit simply does not encourage strategy, it encourages waiting.
A more conceptual issue is the fact that the player has very little control over the most important resources in the game, whereas the RNG element is huge, especially in the early game. In the late game you can remedy monarch point shortage by promoting advisors to a high level if you have a good economy, but early on you just need to be lucky. Getting rid of heirs and praying that you get a better one or relying on save scumming are not good solutions either. It certainly is frustrating not being able to do much while playing well simply by being unlucky. I get the idea that the rulers of a country should have an impact on the quality of your government and that some RNG is needed so that campaigns play out differently, but it certainly does feel weird that in this case a strategy game does not reward you for playing stratecial. I would much prefer a system in which the RNG element is less dominant and the player is rewarded for doing well in certain categories. Here is an example:
What if ruler stats would only range from 0 to 3 in each category and the potentially remaining 3 points per categories could be earned by meeting certain criteria (1 point per criterium):
- Military
being at least at 75% of your force limit
having high enough army tradition (depending on the age)
have high enough prestige (which is mostly earned by winning wars)
- Diplomacy
having high enough navy tradition (depending on the age)
having high enough diplomatic reputation
having enough allies/subjects
- Admin
have less than 5 inflation and less than 10 loans
have less average autonomy than say 40%
something related to institutions or unrest
I am neither saying those criteria are perfect, nor that they are any good at all, but you get the gist of the idea. On the flip side you could also put in mechanics that cost you monarch power, like -1 monthly mil for losing a war, -1 monthly dip for dishonouring an alliance, -1 monthly admin for going bancrupt and so on. In a addition to encouraging the player to think ahead and rewarding him for doing well you could have more interesting tradeoff decisions as well: Do you want to invest in a larger army (which contrary to promoting an advisor has an actual benefit aside from monarch points) and get more military power or do you want to invest in your economy? If such a system is done well you could potentially even see differences between gameplay styles: A very expansion oriented country should do really well militarily and face problems with its administration and at the same time a "tall" country would have plenty of administrative power, but would have a harder time militarily.
Summing up, mana has a tremendous advantage in providing fairly natural bounds attributed to the power of your government/tag in a game that is mostly about expansion. At the same time it also has the potential to dull the game by encouraging waiting and limiting player influence. It all depends on the implementation.
