help me understand, why is multiclassing warlock a bad idea?

OK, long story short - I am NOT min/maxing and will probably multiclass as a way to customize my character but in reading up on the rules it seems that most folks playing warlocks frown on multiclassing them.

I'm looking at starting as Fighter then alternating as Warlock every other level. Lets look at a future state of total character level 6 and compare Warlock 6 vs Fighter 3 / Warlock 3.

Warlock 6 gets 3 cantrips, 2 (3d level) spell slots, and 3 invocations.

Warlock 3 gets 2 cantrips, 2 (2d level) spell slots, and 2 invocations. So by multiclassing I would be missing out on the 3d level spells, but only 1 cantrip and 1 spell slot. On paper, it doesn't seem to be an unfair trade other than missing out on the 3d level spell slots.

BUT, if I multiclass with Fighter and take Eldritch Knight, then at Fighter 3 I would get 2 more cantrips and 2 more spell slots so I'd actually have more cantrips and spell slots than if I had single classed Warlock and the only thing I would be missing would be the 3d level spells (just looking at it from the spellcasting perspective).

So... what am I missing that's so great about Warlocks that folks don't like multiclassing them? It seems to me that their flexibility in casting (refresh on short rest) coupled with their low number of spell slots makes multiclassing them a good way to gain the benefits without losing a lot in the trade.