Limitation

In case the contract is a proxy, which forwards all calls to a delegate, interface detection doesn’t work, which means is_erc20 and is_erc721 will always be false for proxy contracts and they will be missing in the tokens table.
The metadata methods (symbol, name, decimals, total_supply) for ERC20 are optional, so around 10% of the contracts are missing this data. Also some contracts (EOS) implement these methods but with wrong return type, so the metadata columns are missing in this case as well.
token_transfers.value, tokens.decimals and tokens.total_supply have type STRING in BigQuery tables, because numeric types there can't handle 32-byte integers. You should use cast(value as FLOAT64) (possible loss of precision) or safe_cast(value as NUMERIC) (possible overflow) to convert to numbers.
The contracts that don't implement decimals() function but have the fallback function that returns a boolean will have 0 or 1 in the decimals column in the CSVs.