I do think this mechanic will be banned in singles in the same way Dynamax was. The principal difference between this and Mevos was that, altho there were some Mevos broken, each of them were predictable. Not in the specific set, but in it's capabilities. When you see a Medicham at team preview, you already know it's Mega Medicham. Is M-Med broken? May be, but the thing is you know what you are fighting against, you know what its attack can be, what its potential movepool can be, and what can check it. You are fighting a very specific threat; now, if that threat is obviously broken to the point that even knowing its limitations it still surpases the meta by bruteforcing through, that specific individual gets banned.
The problem with Dynamax is that not a single Pokémon is a threat in itself, but each one has the potential to be so. A random Mantine at any given moment can snowball through your team; any random mon at any given moment can dynamax to survive a OHKO; it forces a constant 50/50 scenario every single turn, and this, in a format like 1v1 singles where snowballing effects are much difficult to overcome than doubles, is dreadful.
With this, it is the same. The reward for winning the 50/50 in a given turn determines the outcome of the entire battle. How can you exactly fight against a sweeper that can change its typing to anything else after boosting, rendering its check/counter useless? You have to:
1) Be 100% sure you know what mon in your opponents team will teracrystalize. This is already impossible.
2) Be 100% sure to what type will it change to.
3) Be 100% sure in which turn will this happen.
If you fail a single criteria, snowball effect is comming inmediatly.