No matter how skilled a welder, there are material properties to consider.

In extruded form it would have been 6061-T651:
tensile strength of about 45ksi and yield of 40ksi.

After welding is in the -T0 condition:
tensile strength of about 18ksi and yield of 8ksi.

Eventually 6061 will age back to T4 or so, which is pretty close (70%), but it requires heat treat to get to T6/T651 strength levels.

Compared to the base (original) material, a welded area in aluminum can be a significant weakness that needs reinforcement.