How hampered will a character be with a 16 instead of a 20 in her primary attribute?
The answer to that question cannot be quantified. You can generate a probability table to quantify the hit frequency at 16, 18 and 20, but you can't quantify the thing(s) you got for deferring one or more ability score bumps. That will vary by feat selection, playing style, party composition and DM. Unquantifiable things. And who can say in advance which moments will be the sweetest and why?
I might split the difference. Take the ASI at 4th level; get that primary ability up to 18. Take a fun-filled feat at 8th. Max out the ability score at 12th. I say "might" because so many variables will come into play to influence that calculus, not the least of which is multiclassing before 12th means the ASI/feat progression gets pushed back. In any case, we're in subjective territory.
If I was building a controller wizard, I'd pump Int at 4th and 8th. Spell slots are a precious commodity and spell DC is crucial. For a cleric/fighter, deferring a stat bump is easier to justify. Healing spells never miss. I could see sitting at Wisdom 16 for a while. The primary fighter attribute will probably never get higher than that.
For a middle case, one ASI at 4th would keep me contented. It depends on what you are after, what makes it fun for you. D&D works with a 14. And the DM adjusts the combats to challenge but not destroy the party. Don't tell anybody.
feats are way more appealing to me than ASI
Yeah, you and me both. And fun is where it's at. When it comes to building characters, I consider myself an unapologetic powergamer, but I powergame to build a character that will be the most fun to play, not a character that will do the most damage.