I think that blended teaching is perfectly sensible, where it is more advantageous than not. However, I think pure online teaching can be difficult and at times it is much better to be together in person so that things can be explained in a more interactive way. I also think that university learning can be moved to a blend of online and in person much more easily than secondary school teaching, as lectures in particular are fairly well-suited to online learning since they are naturally quite impersonal due to the amount of students in the room. One-to-one meetings just for discussion can also be done online fairly well (I've had a call with my personal tutor via Teams and it felt the same as in person), though I think seminars are best done in person.
When it comes to secondary schools, I do think that this is more difficult to be done online (either fully or partially). Since this is compulsory schooling, the pupil's attendance has to be monitored which means that a structured day is key. It could be achieved through the pupil having to do their work in a virtual classroom all day every day, but I don't think it's healthy to put children though so much screen time. I had a virtual internship last week where I was on my laptop from 9am-6pm and later in various online meetings and doing work on laptop software, and it was quite heavy on the eyes! I also think that completing secondary school work from home could be fairly isolating and feel less supported. University students are geared for independent learning, but those under 16 are not used to it and don't have this skill as well-developed. Finally, I'd say that there's also the risk of poorer pupils not having proper access to the equipment needed and of pupils at home being more subject to whatever is going on at home (e.g. caring responsibilities, being constantly interrupted/distracted, unstable home lives, etc.), which widens inequalities and which we have already seen as being issues under lockdown.