E61 thermosyphon

The E61 is still extremely popular in home machines because:

It looks beautiful (to most people).

It is well understood.

It is a very simple and reliable design. For example, it incorporates a mechanical 3-way valve with no need for a separate solenoid.

Parts are readily available from multiple sources.

There are many upgrades and accessories available including competition filter baskets, shower screens, and flow control devices.

In spite of its technical limitations, it still does a pretty decent job.

https://www.reddit.com/r/espresso/comments/sssbnu/another_question_what_are_the/

Some says a pre flush helps， having a PID even better. engineering espresso  

But from HB, first and second shot is not that stable. Between 90 to 96 deg c 

And HX requires a flow restrictor ( o ring)to improve the stability mobile01

Managing HX brew temp ... With a cooling fan

Pros:

Temperature Stability

Durability

Wide Availability

Cons:

Warm-up Time

Flush Before Brew

Size and Weight

https://thebrewratio.com/

How about saturated group?

LM GS/3（saturated grouphead) does it much better bilibili

SCA protocol says 93 +/- 3 deg c

From Ascaso:

T group is actually refers to thermoblock type

Warm up simulation using openFOAM, HB

Reference 

GS3

Disassembly and cleaning