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 the 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.