Spike and ferrule
A spike passes through the front of the gutter, through a spacer tube called a ferrule, and into the fascia board. It is quick and cheap, and it holds by friction alone. Fascia swells in Kansas City humidity and shrinks in winter dryness, and each cycle enlarges the hole a fraction and walks the spike out. Eventually you see nail heads standing proud along the fascia and the gutter sagging between them.
The other drawback is cosmetic but permanent: the spike passes through the visible face of the gutter, so every fastener is a hole in the front of the run.
Hidden hangers
The hanger sits inside the trough, grips the front bead, and is fastened through the back wall of the gutter into the fascia with a structural screw. Three things improve at once — the screw holds by thread rather than friction, the load is spread through the hanger body instead of concentrated at one point on the gutter face, and nothing is visible from the ground.
Spacing still decides it
Better hangers at the wrong spacing still sag. We tighten spacing on long runs, on north elevations where ice sits longest, and under valleys where a large roof plane concentrates into a short section. Fastener type and spacing are one decision, not two.
Converting an existing system
If the gutters themselves are sound and the fascia behind them is solid, conversion is straightforward: pull the spikes, fit hidden hangers, re-pitch the run, and seal the old spike holes in the gutter face. It is repair-scale work.
The exception is fascia that has gone soft where spikes have been working for years. Screwing into compromised wood just fails again, more quietly. That wood gets addressed first — which is why we look behind the gutter before quoting a re-hang, not after.