Telegram is good, as you mention, to be relatively private in groups/chats/channels without a need to expose neither your phone nor even a nickname (unless you live in autocratic countries — will come to this later).
But it comes with costs. First, their p2p communication is not e2e encrypted by default. Not to say that all comments/group chats are not encrypted too, unlike let’s say WA.
Second, Telegram API. It gives too much information. You can do a lot with it: read history, track changes of usernames, etc. For example, it is quite easy to obtain an internal user ID and there are black market services and databases where they promise to connect that ID with phone number if that account ever had privacy settings switched off in the past.
Claimed that they kind of scrape all accounts and pair ID for those where privacy settings set poorly. Even if you change it later — your internal ID and that scrape will state forever.
Third, Telegram was funded by Russian government since Durov had issues with SEC. He raised money from different Russian state-owned banks like VTB, issued bonds which are traded in Saint-Petersburg stock exchange, and even take some money directly from Russian government though a Qatar proxy-company. Not to say, that there are cases when TG was involved in criminal charges against people (the most famous one is story with Ryanair plane being forced to land in Minsk to arrest Lukashenko’s critique) and it was never directly addressed and explained by company how exactly those people was caught and how company protect against “SIM card replacement” cases (Signal at least inform me everytime my peer logged to new device).
Selecting between Signal with AFAIK no known cases of charges in dictatorship countries like Russia, funded by non-profitable charity, and TG without default e2e encryption, public API and Russian-state funding, is quite obvious for me.