It's technically slim possible to guess or brute-force a private key or phrase, but the possibility of doing this is very small. Private keys and phrases are generated using very hard algorithms, and the number of possible combinations is large.
For example, a private key for a Bitcoin wallet is 256 bits long, if I guess correctly, there should be 2^256 possible combinations that make up one wallet.
Even with a powerful computer, it would take many lifetimes to guess the correct combination.
In conclusion, the possibility of someone else guessing your wallet is very slim, so going into that scenario will end up wasting your time. If you are afraid someone will guess your private key, you should remove that from your mind and focus mainly on protecting your wallet from outsiders who will gain access to it. Using computer power to search it out is nearly impossible.