Many people have already explained that there are no cons with ISA, but to give you a better perspective on why the government runs this scheme, it is important to understand how they get tax revenue. You probably noticed that pension schemes have been “nerfed” significantly under Osborne’s watch due to no short-term tax revenue from them (i.e. the government will be getting tax revenue when you are old, not today). ISAs, on the other hand, give the state tax money now, because you pay tax on your way in, not your way out. This is why ISAs have been made slightly more lucrative in the recent years, to motivate people to slow down on their pension contributions and incentivise them to pay taxes now, whilst making their future gains tax free (win-win strategy).
In any case, pension, as a saving vehicle, is still (arguably) better than an ISA, but that is a different topic altogether