Replacing one currency not under the government's control with another doesn't seem to achieve all that much, and arguably, picking a deflationary currency at that might be much worse.
Slight correction: They are replacing one currency that is not under their government's control, with one that is not under any government's control.
El Salvador didn't dollarize due to inflation, as other countries have done (Ecuador or Zimbabwe).
The Salvadoran Colón was never an unstable currency.
One of the main advantages we had after dollarization was the lowering of loan interest rates.
I was too young to have loans at that time, but I recall my parents mentioning the repaid their mortgage in a shorter time because of that.
Also no foreign exchange risk with the major trading partner and the country where 2 million Salvadorans live: The U.S.
The downside of using the USD as a local currency is that the local central bank cannot set its own lending rates (markets will set rates at what the larger dollarized economy in the US is at) and has less control over monetary policy.
Salvadoran here
After dollarization happened in 2001, all loan interest rates went down. My parents paid their mortgage in a shorter time because of that.
That was true even when the Salvadoran Colon was stable for most of its history. It's been very close to 8.75 for the past 30 years.