It's simple: you've never used it before, you have no experience with it and this can lead to a dangerous situation.
It's a tricky drug that most people are ignorant about. It accumulates in your system, a lot of people don't know that it can take up to 2 hours or even longer at time for it to kick in, so they redose. Potentially causing an overdose.
For this reason, we as mods of BDD generally do not recommend methadone for people who don't have experience with it. Remember this is harm reduction.
There are many less complicated and 'safer' opioids to have fun with - codeine (for opiate virgins), hydrocodone, oxycodone, hydromorphone, morphine/heroin, etc.
Leave methadone for those people who are on it daily (MMT). People like that can that massive doses and be ok. I can easily take over 250 mg of methadone and be fine - I'm on 130 mg/day.