What do you think makes Astrologian so strong?

[Heilstos]

It's a very interesting question. I think the strength of the astrologer can be justified by the flexibility of the healing forms and the pure support in the form of a damage buff through the cards (which, however, is called an unimaginative idea by some of the community).

Let's take a look at Diuanalfrom:

Because of the possibilities that hots can put, there is a high HP for the astrologer, although this health output is influenced by OGCDs.

Thus, these OGCDs enable the astrologer to ensure a regulated healing flow. The problem with the story, however, is that players need to understand how these OGCDs pack together. e.g. let the bubble be stacked with CO by the astrologer and therefore no aspected Helios has to be used.

Now look at the Nocturnal form:

This healing form is underestimated by many players! Because in the first place this form should not catch up with HPS output. Rather, this form should negate damage as best as possible (so that it hardly needs to be healed). In this case, too, the nocturnal field (created by asp. Helios) and CO are both stacked together and a larger shield can be built up. If we look at the bubble astrologers it is so that they keep in line for 20 seconds (!!!) there is a 10% mitigation buff (which can be extended up to 40 seconds) on a 60 second CD additionally a healthy AoE heal with Helios.

Flexibility has its costs, doesn't it?

Since Shadowbringers and the neutral sect. the astro completely breaks through the force of the stances. (Only 20% boosted for 20sec and every 120 sec). So that the disadvantage "your stance influences your healing form" is no longer as important as ever.
In Diurnal there are shields that are welcome and in Nocturnal there are hots that support the Co. Healer in both forms.

If you feel you can do everything, where is the disadvantage?

The card system is stressful to play, which is also okay with the strong heal output from the GGCDs.Another weakness of the astrologer is the lack of a second mana regeneration effect the functionality of this spell is so enormous that the mana cost reduction cannot be taken as an integral part of this talent. This could be avoided by the piety of Marteria, which makes much of the community reluctant. However, it is noted for new players.

[Rilifane]

Several reasons:

1. very mobile thanks to fastcasts, almost completely negates uptime loss even while learning fights; you can even ferry mechanics so others can move less and keep better uptime
2. free single weave every Malefic cast, making our oGCD heals 100% free while WHM and SCH have to pay opportunity costs outside of dot refresh or ED Ruin II for SCH
3. least dps loss and more gain from GCD heals thanks to Neutral/ Horoscope/ Synastry IF you actually have to hardcast - which doesn't happen at all in most fights
4. free mitigation from CU on relatively short cooldown (although timing and placement in Diurnal is admittedly more difficult than pushing Temperance)
5. wide array of oGCD heals, some of them extremely strong like Earthly Star
6. cards and Divination are % bases which are prone to power creep and outperform raw dps extremely early; escpecially if said raw dps is prone to dps loss from moving/ healing

AST combines powerhouse free healing with easily sustained dps uptime and least dps loss from GCD heals.
If not for the annoying "targets both healers" mechanics you would see a lot of solo heal parties with AST pop up.

The only minor drawback AST has is the MP economy which can be easily circumvented by melding some Piety or switching to Piety gear and buff food for prog.

Now look at the Nocturnal form:

This healing form is underestimated by many players! Because in the first place this form should not catch up with HPS output. Rather, this form should negate damage as best as possible (so that it hardly needs to be healed). In this case, too, the nocturnal field (created by asp. Helios) and CO are both stacked together and a larger shield can be built up. If we look at the bubble astrologers it is so that they keep in line for 20 seconds (!!!) there is a 10% mitigation buff (which can be extended up to 40 seconds) on a 60 second CD additionally a healthy AoE heal with Helios.
.

Why would you chose to hardcast before the damage just so you have a shield and have less to heal afterwards when you can simply eat the damage (provided that the shield isn't absolutely vital for survival) and let free, strong regens heal it afterwards? Unless you need the shield to survive, it's less efficien and costs you time and MP you could've spent elsewhere.
CI in Noct is crazy good for tank upkeep but the shields from Asp. Helios are probably the least interesting thing about Noct. Not to mention you can plan Neutral IF you absolutely need a shield.