All I know is that generating seed phrase directly on Iancoleman which follow BIP39 standard is safe, also generating that on an open source wallets like Trezor that follow the BIP39 standard is also safe. If you do not generate the entropy carefully, you can make mistake, you will need to throw coins 256 times to write down the binary outputs, and the result should be as exactly as the coin is tossed. But, I will still again recommend you to generate the seed phrase directly.
The Ian Coleman website is using javascript, in a browser to generate entropy and your seed... I'm not sure I would be comfortable calling it "safe". 
It's certainly a useful tool, and open source, and nothing obviously malicious about the code it is using... however, none of that proves that it is cryptographically "safe"!
Also I realized that the seed phrase can be more than 24 words long?
Not if you want to maintain compatibility with the BIP39 "standard"... The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
Entropy is in multiples of 32 bits, min length of 128, max length of 256. So, to be strictly "BIP39 compatible", your seed should be 12, 15, 18, 21 or 24 words long.