400 digits vs 24 mnemonic phrase

[jeffytune]

There are many popular wallets which have quite secure seeds and generate mnemonics which are quite difficult to crack in reasonable time.

But I just want to know if I generate private key from my own 400 digits (only numeric) then how it would be less secure than 24 mnemonic recovery phrase?

[Casy]

There are many popular wallets which have quite secure seeds and generate mnemonics which are quite difficult to crack in reasonable time.

But I just want to know if I generate private key from my own 400 digits (only numeric) then how it would be less secure than 24 mnemonic recovery phrase?

With 400 digits there are (10)^400 = 1e400 possibilities, with 24 mnemonic recovery phrase (assuming only small letters and an average word length of 8 characters) there are 26^(8*24) = 5e271 possibilities. There are even less as here I count all possible char combinations, but in an mnemonic, there are only words that make sense.
But just as a rough estimation, it should be ok.

[jambola2]

Even at ~13-15 chars, the likelihood of someone bruteforcing it is near zero.   
Heck it might be even closer to 10-12 chars, because nobody would try even if 10 chars might be almost possible.

The only point of weakness is you. Where are you getting the 400 digits from? Is it truly random? If not, you might have some weakness someone can exploit to figure out the numbers.
Where are you getting the 24 words from? Are they a sentence? Are they closely related words? Did something generate them from you, or did you think of them? If you use a sentence from anywhere, it will instantly make it much easier than bruteforcing.

The truth is nobody is going to bruteforce any password >15 characters or any mnemonic >6-7 truly random words anytime soon. Don't overdo it, it doesn't increase the likelihood of safety, and to the contrary, may decrease it (you might forget the password, or you might write it down somewhere and someone might find it it). Good luck.