Full node and SPV client differs in terms of the kind of security they can provide. Full nodes verifies each and every block, ensuring that it follows the network rules. SPV clients trusts the server to be providing accurate information about the network and they will take the longest and highest difficulty chain as valid.
SPV client will consider longest chain valid without verifiying is it valid or not. So an attack factor is that some evil miner with big hashrate could create an invalid chain and beat the number of blocks on valid chain to fool SPV clients. But if that evil miner has the ability to create longer invalid chain he has the ability to make Selfish Mining attack also. Which will be much more profitable for him with the block rewards.