Strange that this mixer doesn't provide letter of guarantee BEFORE i'll send coins.
A Letter of Guarantee can only be used if the mixer transfers mixed funds to a certain address. ChipMixer gives you private keys, and it's not possible to announce them before you deposit your funds.
If means in case of selective-scam (large deposits) nobody will able to prove that address really regarded to chipmixer.
Correct. That's why they're highly trusted here.
PS
Exchanges or casinos don't provide you with a signed message before depositing either.
In this case Letter of Guarantee should be a
proof, that generated address regarded to chipmixer, nothing more needed (i'm talking about before deposit step). What will you do if chipmixer will change deposit address silently after you make a large deposit? How will you prove that address you saw on page was generated by chipmixer? Post a screenshot? Lol.
PS
i trust exchanges much more than anonymous lagging service which even can't answer customers for few days, obviously. Strange that you comparing coin-laundering service with legal stuff, lol.