Pensions have some pretty big advantages. They might not be the best thing for you, but make sure you know the rules and think it through then make an informed decision.
Benefits
1.Employer contribution matching - if you're lucky enough to get this, it's probably worth maxing it out since it doubles your money.
2.Marginal tax rate: you pay tax on your pension at the time you tale it, rather than now whilst you're earning. If you're in a higher tax band now than you will be in retirement, that's beneficial (and fair - since that's when you get the income!).
3.Tax free lump sum: you can take 25% of your pension tax-free, up to a maximum of £268,275. That means that if you'll be a basic rate tax payer in retirement, then your tax rate will be 15% rather than 20% (If you're a higher rate payer in retirement you'll pay 40% instead of 30% for up to £450k - but you'd need a huge pension or a lot of other wealth to be paying enough tax to get the whole lump sum at the higher rate). It's likely that this benefit will be reduced or go altogether soon. (Warning: be very careful about what happens when you take your lump sum - speak to an IFA!)
4.Interest or capital gains are tax free.- this does make quite a big difference, especially if you're a higher band tax payer and most of your investment growth gets stolen by the government! If you haven't maxed out your ISA allowance each year, then putting it in a S&S ISA will give you the same benefit
Negatives:
1.You can't take anything out of your pension before 55 currently, and that's going up soon. That means you can't easily protect yourself against whatever insanity they bring in
2.Like you said, they can change the rules at any time.
a) Loss of the tax free lump sum is likely
b) They might start charging NI - but you've already paid that on the way into your pension so you'd be paying it twice! (Totally unfair, but I have no doubt they'll spin it lie as some sort of tax break)
c)the other likely change is on tax relief on the way in. Beware: that will completely change the calculation of whether it's worth adding more into your pension, depending on exactly what they do.
3.The other thing Reeves has hinted at is insisting on S&S pension investments being at least 25% UK equities. You will have your own opinion on UK equity performance, if that does come in (and the wider risk of government interference in investments you can't withdraw until age 55/56)
Personally I think the advantages currently outweigh the negatives - especially if you can either get matching employer contributions and/or you'll be paying tax at a lower marginal rate in retirement than ypu are now and/or you've already maxed out your ISAs.
State pension
It's a bit of a no-brainer to keep your NI contributions up if you can. Whilst state pension might reduce, it's such a huge benefit (worth £250-300k) that you're unlikely to lose it all in even if they introduce means testing.
You can get your stamp for lots of reasons: being on sick pay from your employer, claiming benefits, maternity leave etc. If you're self-employed, do try to take enough earnings to get your stamp. If you do miss a year, you can pay it back, but think about whether you need to: you only need 35 qualifying years, and you get a few for free (you can look up your NI record online). Although I wouldn't be surprised if the number of qualifying years went up at some point (35 years is less than most countries)