Every big computer system is vulnerable to cyber attacks, especially a high profile one associated with security. I don’t see that as a unique issue with being in the EU, but quite prepared to be told otherwise.
As for Labour, they only had two choices really. Either they go all-out for a new referendum on rejoining, or they accept the deal we have. Whining somewhere in the middle gets none of us anywhere, especially not at this late date. It’s a shame - and quite telling - that it’s still the Tory extremists we need to worry about.