Lumping AZ and JNJ together seems inappropriate.

The fundamental problem with AZ is that they don't seem to understand how to conduct clinical trials and do data science and so you can't trust any of their numbers. Population level studies on the effectiveness of the AZ vaccine everywhere but the UK would seem to show it is barely effective or ineffective. The population level studies in the UK were largely conducted by the Oxford team that developed the vaccine themselves or their "friends". The head of the team is politically connected. The UK population level studies are almost certainly complete bullshit because during the period when the vaccine supposedly reduced Covid deaths, a draconian lockdown was instituted which certainly had an effect as well, and despite this excess deaths went up by a bit more than half the supposed reduction in Covid deaths. It's not clear the reduction in Covid deaths was any more than would be expected with just the lockdown. The well publicized UK studies are in my opinion, more likely than not, massaging of the numbers to make the Boris Johnson government look good, especially relative to the EU post-Brexit.

The Oxford Team and AZ essentially openly admitted to doing p-hacking at a press conference and insist that the vaccine should be used if the results are significant only in any sub-sample. The Oxford team in particular has had a history of claiming fake good vaccine results for other diseases that aren't prevalent in the West. Their history of doing this may not be well known in the West but there have been a lot of problems here. Each problem seems to have had the same cause, p-hacking so they could claim they had good results and get research grants. It's unfortunate in my opinion that a lot of UK and EU policy was based on things these guys said, for example delaying the gap between shots etc.

For a vaccine with the actual rather than claimed level of effectiveness of the AZ vaccine, the appropriate level of acceptable side-effects is very low. You only approve a drug if the cost/benefit makes sense.

The JNJ vaccine by contrast seems excellent. The top line numbers are not as good as PFE/MRNA but you have to look deeper than that. PFE/MRNA cause side effects including fever etc in 30-40% of people after the second dose. Many people, especially those who've had Covid, get side effects after the first dose as well. JNJ does not prevent mild Covid in 35% of people but the symptoms of this mild Covid are comparable to the side effects from the second dose in the 35% of people who get side effects from PFE/MRNA. In a world where getting exposed to Covid is inevitable, like in the US or Europe right now, it probably doesn't matter which you get, you have to deal with a 30-40% chance of a week of flu like symptoms. However, in a world where Covid is rare, hopefully soon, with JNJ you end up with less symptoms overall and something just as effective in preventing serious Covid and death as PFE/MRNA. Essentially, if there is only a 4% chance of being exposed to Covid when you are out an about, your risk of a week of fever is 4%*35% with JNJ rather than 35% with PFE/MRNA and it's otherwise very comparable.

The FDA action on the JNJ vaccine may be overly cautious. About 1 in a million vaccinated got blood clots over almost 2 months of use of the vaccine. In a typical year 6 in a million get blood clots anyway and the frequency seems to be a bit higher in colder months. Given the population groups most vulnerable to blood clots, based on the data currently publicly available, the frequency of blood clots does not seem to go up with the JNJ vaccine either in the population as a whole or in any subgroup. I suspect the FDA action would not have been taken if it hadn't been for the mixup at the Baltimore plant. They likely want to triple check production facilities and the vaccine in the logistics chain because it seems some contaminated vaccines did make it to vaccination sites and may have even been injected. If you got a vaccine with the ingredient switcheroo, it's not unreasonable to believe you would have a higher risk of blood clots. There is no actual study of this, and you would have do a real experiment to make a solid claim but given everything we know increased risk from a vaccine with the ingredient switcheroo is more than just possible.