former HVAC tech here. Sorry for the wall of text but here's most of what I would do as a seasonal change-over inspection:

Every year, a full visual inspection of the heat exchanger, especially important after 5 years of life. We're looking for the first signs of perforation, and/or cracks. A "smoke test" isn't sufficient.

Visual inspection of ductwork take-offs on plenum - leaks, seals, etc. Same for system cabinet.

Check the safety switches -- roll out/limit sensor (millivolt), draft induction (exhaust blower) pressure switch, cabinet switches, etc.

Clean the flame sensor.

Visual inspection on the ignitor. Cracks = replace. Check values on start capacitor for blower motor. Replace if > +/- 15% of spec.

Blow out P-trap on condensor drain (only applies to condensing furnaces, of course -- the >85% efficiency models, but that's most of what is installed these days). Oil blower motor and draft induction fan bearings. Observe flame color.

After 10 years, or as needed, check exhaust with combustion analyzer for proper C0/C02 range.

Observe 2 startup/cool down cycles for proper operation. Look for proper ignition, flame color/depth/shape, listen for odd sounds that would indicate issues (vibration, flexing, etc.)

Replace the filter unless it's obviously new. (btw, This filter is there to protect the unit from dust/lint, not there to clean the air in the house.)

Replace CO detector battery Not just "ask homeowner to do it" but do it myself. Replace detector out of date (they only last 10 years or so). This is mandatory if you are operating a fuel burning appliance. And Remind homeowner to change smoke detector batteries.

Present a written report of all findings. Compare to last year's report -- look or trends. Keep copy of new on on record at the shop.

This takes about an hour. If your "HVAC Guy" just shimmies into your crawlspace or attic for 15 minutes before climbing back out and saying "everything looks great, see ya in the spring" you're risking your safety and being cheated. Most don't do the whole job. Also, what most shops call a "tune up" isn't anything of the sort; at best it's a cursory visual inspection with gauges on the A/C unit and a check of the start/run capacitors. Most don't even bother to oil bearings.

Every shop is different. There is a wide spectrum of hacks and pros.

Thanks for the info. :)

So a forced air furnace is basically the same as a boiler where you just call a technician to do your "yearly servicing?"

The company that services my boiler spend well over an hour on it, so I'll assume I'm not getting ripped off there?

It depends on the model, quality and features. The one that came with my current house was very finicky and needed a fair amount of handholding (including pouring water into a little tube at the start of the season - something about a proper vacuum). I replaced it a few years ago with a new, near-top-of-line Trane with an air filtration system and a whole house humidifier. This one only needs to have the filter vacuumed monthly (three-stage electrostatic filter), and to have the humidifier water panel replaced once a year. Otherwise it’s basically no maintenance.

Any forced-air system has ductwork, however, and that can get filthy over time and need cleaning.

I don't know about furnaces in general. It's just that the old place's steam heating never really gave us any problems, whereas our furnace has needed repairs multiple times over the past few years.

Maintenance was bleeding the pipes occasionally, which I generally found to be less of a bother than replacing filters.