Yeah, predecessors said almost everything. The conclusion that can be drawn was also already mentioned - different experiences with different brands and models, or every boot doesn't fit every foot - advice: try as many boots as possible to find out which ones fit your foot.
Things that haven't been pointed out yet.
1. When trying on boots, you need to wear a hiking sock (your own or the one in the shop - every good one should have them), to get a real picture of the (dis)comfort of the boots. It's logical that the boot fits with thin socks, but when you have a thicker hiking sock on your foot, it might already pinch.
2. When trying on, do the thumb test, which you push behind the heel; at that point your toes should press against the front part of the boot - that is, the boot should be larger by the thickness of 1 thumb. Identical to this test is the test with the inner insole, which we take out of the boot - put the heel to the end and see how much space remains in front. This is very important on descents because with proper lacing of the boots there won't be pressure of the toes on the front and consequently fewer blisters.
3. When trying on boots, pay attention to how the boot 'holds' your heel; if the heel lifts, you have too big a boot on - blisters!!! or the last doesn't suit you.
4. Given the increasing prices, for those who REALLY use mountaineering boots a lot, it might not be superfluous to consider if resoling the sole is possible on the chosen model. If we calculate the lifespan of a mountaineering boot is around 400-500 working hours - someone who walks a lot, especially in rocky terrain, 'wears them out' in a good year or two, price of quality ones from good 100 EUR or 200 EUR upwards, sole replacement around 30 EUR... Replacement is not possible on those models that have the sole rubberized-glued also on the side of the boot - which are almost all lighter hiking boots.
5. We must be clear about what purpose we are buying boots for - whether they will be mainly for walking on soft forest paths or perhaps for winter high mountains or glaciers - and choose a model suitable for the main activity. Different models of mountaineering boots are for different purposes and differ greatly from each other, also in price. Compromises are usually not the best solution - usually people who visit mountains both in summer and winter conclude to get two pairs of mountaineering boots - winter and summer ones, plus some low ones for forest terrain.
5. A lot depends on the socks we use. Socks must not be too big or too small and above all with as few seams as possible or without seams. Blisters usually form right where the seam rubs against the skin. So when buying mountaineering boots, it's wise to also buy quality hiking socks.
5. Sad truth - you also need some luck; the boot might be completely comfortable in the shop but after ten hours of walking it might blister us. It's best to minimize this possibility as much as possible (advice above, help from someone who knows at purchase, prior use of boots at home in the apartment...)