Yes, sadly on many projects.

My top reasons, in no particular order:
1) Mini prep.
Removal of integral bases that encase the models' feet like Mafia-style concrete shoes, mouldines that border on mould slip, models that just don't fit together properly, huge amounts of putty work, spindly parts that bend/break but are nightmarish to pin, etc. I actually enjoy mini assembly (it's my favourite stage I think, especially when a coat of primer coalesces your efforts into a "ready" model), but some tasks are just so much work that it saps all my enthusiasm.
2) Mini material.
I just don't get on with some materials at all, and I find others to vary greatly. The PVC that Mantic uses is just not worth my effort, even though I acknowledge it has some advantages from a modelling point of view. The tin that Privateer Press uses is so hard, that drilling the feet for pins blunts my drill bits awfully fast and after doing a squad of models I usually end up with blisters on my fingers (even though I use carbide drill bits). Finecast resin is, er, well I avoid it like the plague.

3) Extensive conversions.
I like building models, and I like converting. I do sometimes over-commit to tasks though, and extensively converting a big block of models is very hard going. (By "converting", I don't mean bits-swapping; I mean extensive cutting, pinning, puttying, etc). I fall into this trap when I like the idea of a force/faction/army but not the models, or the models are limited/not equipped as I'd like, or even simply that models are not available.
I have a WHFB Chaos army that has a block of converted Chosen (20nr), a block of Warriors with great weapons (18nr, all converted weapons and heads), a block of Warriors with halberds (18nr, all converted weapons and heads again), and a block of Marauders (20nr, all with converted great weapons, head swaps, and custom command). As these are based on the old 5E/6E/7E plastics, they are a lot of work anyway regardless of the additional conversion work; the above additional conversion efforts have stalled this project hard three times in as many years now.

4) Boring or difficult colour scheme.
I find that some "simple" schemes are often very boring to paint, and that many "interesting" schemes take me too long to finish. I like painting, and can even batch paint around 20 models/pieces at a time well enough, but I lose steam quickly. Slightly tangentially to this point, I also find excessive details on models also bog me down hugely, and result in getting models finished becoming a very drawn-out process.
I have a Black Legion Chaos Space Marine army, all kitbashed and converted, cleaned, primed, ready to go. Started painting 5 models, and my enthusiasm just left me after only three sessions. I got the models pretty advanced, but the scheme was just missing something that I couldn't put my finger on, and I fell out of love with them. I think this is one I need to revisit with fresh eyes in the future, and probably need to add more colour to them somewhere.
My first Antares project (an all white C3 force) was another frustrating experience where a gritty primer finish, soft model details, and a very tricky colour all ended in abandoning the project. Incredibly though, I lost enthusiasm just
after completing all the white...

5) Project size.
Tricky one this, but generally if a project gets too big (number of models), I get overwhelmed. If I curtail my tendencies and focus on just a little force, I feel disappointed at the end that my efforts didn't end with a playable force, or a force that after a few games feels utterly stale. I really don't think I've ever hit my balance with this, and many projects remain very unfinished as a result.
The closest I came to "success" here was with my old Malifaux crews - the low model count, individual models, and complex gameplay has meant that the crews I did (mostly) finish have been fun to play. The models are also not too complex in terms of details to paint, and most are reasonably simple to assemble.
6) Perfectionism.
"Perfection is the enemy of done"... Or so I've heard!

I know what I'm capable of achieving, and for some reason I feel that producing anything less than my absolute best is not worth doing, even though "my best" is a supreme amount of effort/work. I'm working to de-program myself of this mindset, but I find it very hard. I realise that I am likely to only ever paint a given set of models once, since I have so many other things I want to do as well, that I'd never have time otherwise. I therefore feel obliged to do the best I can, even though it results in lots of unfinished things, all abandoned in frustration anyway. It also means that I don't usually play many games where everything is properly finished, which for me does reduce my overall enjoyment. I last had fully-painted and finished armies when I was in my mid-teens, and I cut so many corners and made so many poor bodges on them, that I just feel sad when I revisit those old minis. Then again, they
were done...
An honorary item#7 would be Kickstarters. Too often a KS finishes delivering loooong after I have moved on to other projects, and then results in most of the list above when I do come to tackle it, that I've just stopped participation in all of them now.
The last "big" KS I did was the Mantic Warpath push in 2016, and I found the entire experience so utterly dissatisfying at every level, that I was put off ever doing another one again. (...And Mantic aren't even all that bad by KS standards!)
So yeah, you're absolutely not alone!

One thing I've learned is that it's important to be honest with yourself about whether some things are simply just not fun for you, and to openly consider what options there might be to help you overcome the sticking points in a project. For example, do you have a friend or a professional service that could do all the spears for you? If so, it might be worth approaching them if it lets you get back to what you enjoyed about the project without the part that frustrates you.
And what about those that present a new iteration each three or four years and the models that you painstainkingly collect and paint become redudant? (Yes, I am looking at YOU, GW!) That was one of the reasons I stopped playing 40K altogether and went back to historicals only.
Eh, I find that sometimes accusations like this are a bit unfair. If you had a squad of 10 Tactical Space Marines, with a common heavy weapon, a special weapon, and a modestly-equipped sergeant in it, then I think that's been "legal" in every single version of 40k's nine editions. And the same is also true of most of the other units that I can think of too... Only if you built something that was specific to a particular edition (like, say, a Space Marine Command Squad, all on bikes, and all with a specific weapon loadout), or you bought a large number of a specific unit type for a themed force, would you maybe have some problems with using them in a different edition. Even base sizes is a bit of an overwrought argument, as GW have always said that you can keep models on the bases that you had them on originally.
Arguments about the games' overall balance, and also regarding specific unit utility, are all valid IMO though. (That said, probably a discussion for another thread!

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