To be honest, welding is one of those awesome-looking skills that gets really fun when it works. Sparks flying, the metal coming together into a nice bead, like if you lined up all of your tiny coins. But in real life? Welding can be like barbecuing in the rain: functional, however, not always your ideal option.
1) Warping and weakening of thin metal
The heat is what makes welding work and also why it causes problems. It is even more important on thin stuff, so if you are repairing car panels and light fabrication, they can distort (twist or buckle) and will burn through very quickly.
2) It is not always easy to mix materials
Wish to chemically combine steel with plastic, timber, rubber, and composites? After all, welding is likely just going to not care and answer no. Joining two different metals can sometimes be challenging without a different method. When you need a Metal bonding adhesive, consider https://www.ct1.com/product-applications/metal-to-metal-adhesive
3) It’s dirty (and not just the sparks)
Of course, welding means grinding, cleaning, and repainting afterward. You usually require additional work if you want a nice join that is not visible.
4) Safety and access could be a pain
You need appropriate PPE, ventilation, and a risk-free configuration. Not to mention that sometimes you can’t even get a torch into those tight corners unless half the job is taken apart first.
5) Not necessarily the cheapest solution
Usually, if the repair is small enough in scope and minor enough that there are significant requirements for equipment and expertise to fix it with fewer materials but more effort involved than perhaps an actual mechanical fixing or even a modern adhesive could give, all sorts of worthy repairs.